Hello guys, and welcome to my guide for Jayce, the defender of tomorrow. This is my first guide I have ever written, and I beg you, PLEASE do NOT down-vote without trying it out first. Jayce is most likely my favourite top-laner, and I found, a little bit harder to pick up at first, thanks to his extended skill set. So if playing him for the first time, do not pass him off. As I said before, Jayce is OP when played correctly.
So, lets get started:

PRO'S:

1. Versatile
2. Has only one counter
3. High damage with full combo
4. Decently tanky
6. Gets ultimate at level one
7. Even if you fail in your lane,
you are still quite effective late
game.
8. Ranged and melee

CONS:

1. Countered by Yorick (But then again, so is everyone)
2. Not as effective if combo is interrupted
3. Susceptible to early ganks
4. Mana problems early game

Once again, standard AD offtank runes. Self explanatory, but I will explain anyway.
AD for damage. Armour and MR to reduce damage taken and increase tankyness. Feel free to use scaling MR glyphs rather than flat if you prefer.

Flash
Flash is vital for most champions in the game. Allows you to chase, escape, reposition etc.
By far the most useful and helpful summoner spell you could take.
Barrier
Barrier helps me so much as Jayce . Every game, it saves my life at least 3 times and gets me a couple of kills. It allows you to survive DoT, delayed damage, direct damage, and tower damage. It is so useful, I take it almost every game. Sometimes, however, I will replace it with Ignite or Teleport , as you will see below.
Other spells you could take:
Ignite
Take this against champions with healing, eg. Volibear, as it applies grievous wounds to prevent healing. This is also a very good choice if you prefer this spell to barrier, though in my opinion barrier is a better choice.
Teleport
Useful for ganking, recalling and getting back fast, and getting around the map quickly. A viable replacement to barrier or ignite.
Exhaust If for some reason you want this, take it. But I do not recommend it, since you are not a support. It is 'viable' on most top laners (Allows you to chase and run) but I feel other spells bring you many more benefits. Taking this spell against an AA oriented champion (Eg.
Fiora) is an option, however, as it slows their attack speed, which is their mane source of damage. I would still prefer Barrier personally, though.
Ghost This spell is NOT RECOMMENDED although, if for some reason you are completely against flash, take this instead.
NON-VIABLE CHOICES: Clarity Ok, yes you have mana problems. BUT: A)Blue buff. B)Tear of the goddess C)Potions D)You are not called Cho'Gath and you are not a support. Do not take this.
Heal You are not a support. If you like to healbait, buy an Elixir of Fortitude.
Revive No. Just, no. NEVER TAKE THIS.
Smite You are not Jungling.
Cleanse Waste of a summoner spell slot IMO. If you really want this spell, simply buy a Mercurial Scimitar. Simple solve.

First up, I will go through the sequence. Then, the abilities themselves.
All of your spells are important, so I like to max them pretty evenly, though I do prioritize in this order: Q-E-W-R. I level up your R last, since I do not see many benefits of levelling it. Q first, as it is your primary poke, then E as you use this as your main damage, escape, and combo. Finally, W, as half of it is pretty useless, and the other half is only useful in 1v1s, I find.
Right then, ability explanation:
Mercury Cannon / Mercury Hammer
This is your ultimate. You get this ability at level one, and it does not require a skill point.
Upon activation, it switches your form between Hammer and Cannon stance. Your basic attacks are melee in hammer, and ranged in cannon. You also receive a different ability set when in each stance. Your passive is also centred around this spell. Also, after changing stance, your next AA has bonus effects. After switching to Cannon Stance, your next AA your enemies MR and Armour is reduces by a %. After switching to hammer stance, your next AA deals bonus magic damage.
Hextech Capacitor
This is your passive. Very simple; Every time you use your ultimate, you gain a small boost in speed and can move through units for a short time.
To The Skies! / Shock Blast
Your Q. In hammer form, it launches yourself towards target enemy, dealing damage and slowing in the area. Note that you land where your enemy was when you cast the spell, so if you use it and he flashed, you will appear at his location BEFORE the flash.
In cannon form, it shoots a small orb that explodes on impact. This is a high damage skillshot, and your main ranged-poke.
Lightning Field / Hyper Charge
Your W. In hammer form, it has both an active and a passive. The passive is this: As long as you have this spell and you are in hammer stance, each AA restores mana. Use this to get your mana back up. The active deals magic damage in a small area around you. I very rarely activate this, since it scales off AP, however it does deal decent damage if standing in the middle of a teamfight or minion wave.
In cannon form, it increases your attack speed to the max for the next few seconds/3 AA's. This is an extremely powerful ability to use when: Trading, 1v1ing, or turret killing. It also has a VERY good synergy with Sword of the Divine's active, three instant high damage crits, OP, yes?
Thundering Blow / Acceleration Gate
In hammer form, it knocks back and deals magic damage to your enemy. This skill deals more damage the higher maximum health your enemy has. In lane, your main close combat harass combo is: Q+E+AA+RUN. This deals high damage, and as you knock them back immediately, they cannot usually do much damage to you back, so that combo is very good for trades.
In cannon form, it places down a gate that increases your movement speed when you walk through it. Also, if you shoot a Shock Blast through the barrier, it travels further and faster, and deals more damage. E+Q combo in cannon form is your main poke.

Early game (Levels 1-3)
At level one, you main objective is to use cannon form to kill creeps quickly, and get level three, perhaps using one Q to poke. Level two, you continue to farm, but this time in hammer form, and if the enemy comes near you simply Q+E them. If the enemy deals more damage than you, or attacks you a lot, stay in cannon form and E+Q to poke. Level three: Continue doing the same, be slightly more wary of ganks if you are pushing. If they are pushing you, ask for a gank.

Mid game (Levels 4-7)
Keep farming, trading, and harassing. By the end of mid game, you should have recalled at least once, either to buy items or heal up. Remember to buy wards and potions, and if your mid laner does not have mana, ask your jungler for blue. If you get ganked, this is the main escape tactic: E (Cannon form) and run through it to run faster. Then, switch to hammer form and E the person closest to you away. Then, constantly use R for the movement speed boost from your passive, and keep using both E's whenever they are off cooldown. Towards the end of this phase, you may attempt to go for a kill. This is your main assassination combo: R (So you are in hammer form)then: Q,E,AA,W,R,E,Q,W,AAx3, if they are not dead, keep chasing with AA's. You also have the ability to snipe for the kill with E+Q in cannon form.
Late game (Levels 7-11)
If doing well, begin to roam to other lanes, try to secure dragon with your team, and make some epic plays! Just have fun and do well. If you are pushing your lane, attempt to kill the turret. Mid lane turret is also a large objective in this phase, try to destroy it.
If you are not doing so well, stay in your lane and farm (Remember to place wards!), or stand at the back of fights and attempt to steal/secure some kills for yourself. The best skills you have to ks are: E in hammer form, or E+Q combo in cannon form. Remember, even if you begin this phase with 0-4, you can still easily catch up.

This phase normally involves teamfights, jungling, and catching people out. You MUST stay with your team as much as possible, so you are there when a teamfight starts. In a teamfight, your main objectives are: A)Q+E (Hammer) and W (Cannon) the highest priority target. Note that if they have a champion such as Katarina, you may want to save your E to interrupt her channelled R, Death Lotus. Other examples are: Fiddlesticks and Crowstorm, Shen and Stand United, or even Pantheon's Grand Skyfall or Heartseeker Strike! AFter doing this, you should keep on AAing, attacking, using your E+Q combo (cannon) to catch fleeing enemies or simply to deal damage, and if the fight stays grouped, maximize your damage by using your W (Hammer) in the middle of everyone. If chasing, use the same tactics to chase as you did to run from ganks, but instead of E'ing them away, Q towards them (Hammer). Also, remember that your AAs slow thanks to your Frozen Mallet, which also allows you to catch up. Lastly, you can use Flash to chase, as well as using it to escape. Avoid using it if you are not 99.9% sure it will get you a kill.
During this phase, unless your jungler says not to, you should continuously take wraiths, wolves, golems, and the buffs whenever they are up, unless someone else wants them. You should also group and attempt to steal their jungle. Dragon is also another large objective. If you see someone alone in a lane, attempt to catch and kill them, as they usually have no backup.
If your team is not doing as well, Split Pushing is a good idea. Only do this with champions with good escapes, such as Kassadin, LeBlanc, Lee Sin, or Master Yi.
Again, remember to ward when doing this, to avoid getting caught.
Otherwise, what happens more often is that both teams simply go all mid, in a battle to win fights and push the lane. If teams do do this, simply try to push and kill them as much as possible. Your E+Q combo (Cannon) is very effective for poking and kiting. Also, remember that your E (Cannon) does give vision, so you can use it to see if the enemy is hiding in a brush somewhere.
Further towards the end of this phase, remember about the main objective: Baron Nashor . If a team gets this buff, it increases their overall gold by 1.5k, and greatly increases their regens, AP, and AD. Keep this warded, and take it when most of their team is dead. If you see the other team attempting to take it, attempt to either stop them, or steal it.

At spawn, you have many start options. If you feel you need sustain against a heavy poke lane, start with Crystalline Flask and potions. If against heavy AD (Eg. Fiora, Riven, or Darius, take a Cloth Armor to start, to reduce damage taken. If you wish to begin with damage, take a Long Sword plus potions. Always remember that Doran's Blade is also available, if you want sustain, damage and health., plus a potion. Next, there is this option: Elixir of Fortitude gives you a phage at level one, plus potions. And finally, you could start with wards and pots, for high sustain and not having to recall early to buy wards. I prefer others starts over this one, however.
Your core items give you speed, damage, life steal, health, armour pen, and attack speed. All stats that are simply amazing on Jayce. (NOTE: If you are going to build a Muramana/ Manamune, buy and start charging your Tear of the Goddess straight away). First up, you purchase a Bloodthirster, for the amazing dmg, and life steal. If you are not doing as well, Black Cleaver is also a good 1st buy for a tad of extra HP. Then buy whichever above item you did not buy, before picking up a pair of boots: You have many options for boots: Mercury's Treads if against heavy CC or magic damage. Ninja Tabi if against AD, even more so against AA oriented champions such as Fiora or Aatrox.
Berserker's Greaves are my normal choice, for extra attack speed, and the other final options are Mobility Boots for roaming, or Boots of Swiftness simply for moving around. As for enchantments, Alacrity is good if you like it, although I prefer furor for the chasing speed or homeguard so you can get back to lane faster. Distortion is good if you took teleport with flash, for the reduced cooldowns. Then buy Frozen Mallet; this gives you more dmg, an amazing slow, plus lots of hp. You can get this item second if you feel you need more HP earlier. NOTE: I do quite often buy the bthrister, bcleaver, boots and mallet in a different order, depending on what I feel I need and how I am doing.
Your final two items are situational, and you have many options:

This gives you a large bonus in life steal, damage, attack speed, and an amazing active. A very good item on any ranged AD, nothing much else to say.

This item will make you unkillable. If you buy this item, you will have 300AD+4000hp. Powerful, unkillable. This also gives you a lot of sustain, thanks to the huge health regen.
If I buy this item, I sometimes also go for an Atma's Impaler, I find it compliments this item incredibly well.

This item is amazing if you want straight up damage, and nothing else. This in conjunction with the next item is incredibly powerful.

This item plus your W, Hyper Charge, is OP. Use this active plus your W, 3 instant crits. 1000 damage in 2 seconds, while using other abilities. If you do this when you also have an Infinity Edge... You will be dealing 1500-2000 in a matter of 2 seconds. Wow. When this item is not on cooldown, you will receive attack speed, also useful.

This item is also amazing. More attack speed, AD, movement speed, even lower cooldowns, plus tenacity. If you have bought Mercury's Treads and wish to buy this item, I recommend picking up different boots, as you do receive Tenacity from this, and it does not stack. I do pick up this item frequently, it is very powerful.
Yomuu's Ghostblade
I do not usually get this, however looking at it, I do not see why it would not work. I would pick this up specifically if they are stacking armour, because of the armour pen, but it would likely work anyway, because of the useful active, AD and more CDR. The critical strike chance also increases your damage. If you do get this item, be sure to utilize the active, as if you do not use it, your money would be better spent elsewhere in my opinion.

NOTE: If you are going to build this item, pick up a Tear of the Goddess early, to begin charging it up.
This item completely gets rid of any problems with mana you may have, since it gives mana regen plus an amazing 1000 mana! It also gives a good amount of AD - from its passive, Awe, and its base 20 AD. Be sure not to abuse the toggle, however, your mana will deplete faster than you want it to!

Amazing item on any champion. Decreases damage taken, plus an amazing passive. This passive is the main reason we may take this item: Allows you to die in a 1v1, revive, and go on to kill your target. If you get ganked, you can revive while your allies come, and run. You can flash instantly as soon as you revive, you can use it as time to get your cooldowns off to get away, this item is very useful in all circumstances.

This gives you two things: 1. AD, and 2. MR. If you need protection from those bursty apc's, this is the item for you, no questions asked. If you are against an AP top, an early Hexdrinker might be a good idea, since it builds into this.

Once again: AD and MR. Take this if you are against hard CC, basically gives you a free Cleanse without using a summoner slot. If against an AP with lots of CC (Such as Kennen and you feel this would give you an advantage in lane, buy a Quicksilver Sash during the laning phase sometime. It will build into this item later on.

This item is all-round useful on Jayce. AA deals more damage after using an ability: More damage, thanks mate! The other passive doesn't really count for much since it doesn't stack with your Frozen Mallet, but this is still a good item. Extra AD is always good, AS helps to attack more, and AP will maximize your damage, by increasing the amount of damage you deal with Lightning Field. If your team has a Will of the Ancients, baron buff, and you have a triforce, your Lightning Field will deal around 500 damage to the entire enemy team! You then get the movement speed: Running away, chasing, generally moving around; MS is always welcome. And finally, there is the HP and mana. Mana will allow you to use more spells and the HP simply makes you tankier. Although not part of my normal item build on Jayce, I do very well when I get it. Very good all round cost-efficient item.

This item provides you with: CDR, AD, MS, Crit chance, armour pen, and an amazing passive. If you are in a game where you feel you could make good use of these stats, pick it up! Remember to utilize the active if you buy this item, it is not very cost-efficient if you forget.

Are your enemies stacking armour? This item will completely counter that. Pretend you are a mage and are buying a Void Staff. This is the Void Staff for AD's. Very good for increasing your damage.

On ANY champion, you should ALWAYS try to farm as much as you can. Unless you are playing support, in which case only farm if there is no other person trying to get it.
As Jayce, during the laning phase you should simply sit back in cannon form, and farm. So long as you are not trading or poking, you should be last hitting as much as possible. If you are low on mana, switch to Hammer form and farm like that to restore your mana, using your W's passive. As for using spells to farm, your Q in both forms is good, and W in hammer form at higher levels is also good for farming. Your W in cannon form is good for killing large minions or turrets, too.
Late game, farm as much as you can when in lanes, and farm jungle creeps as much as you can, unless your jungler objects. Use all your skills to farm jungle creeps: I find this combo is good: (Hammer Form) Q+E+W+AA+R+AA+E+Q(+W on red/blue, otherwise it should be dead). It may die faster than this (It should do, a lot faster) at endgame.
If you find you are low on mana but high on HP, and do not want to recall, simply find a jungle camp, and, in Hammer form, hit it with as many AA's as you can. Your mana will go up pretty fast, if you do not use any spells.
During laning, you should decide what you would like to do early: Push hard, and attempt to kill the turret. Freeze the lane. Or, let the enemy push. Against enemies that counter you a lot (Eg. Yorick) I would recommend letting them push, against medium difficulty champions (Eg. Darius), and against easy champions (Eg. Teemo), push hard. Remember to use wards, plus pink wards against junglers who use stealth, such as Twitch, Evelynn, Kha'Zix, or Shaco. How to do these things:
Freezing the lane:
Do what your opponent is doing. If they just last hit, so should you. If they attack them as much as possible, so should you. This should make the lane roughly even, and should stay in the position you freeze it in.
Pushing:
Kill as many minions as you can, as quick as you can. Beware of ganks when doing this.
Letting them push:
Do not farm until minions are within turret range/close to turret range.

Late game, you may find split pushing an option. WARNING: A L W A Y S WARD THE AREA AROUND WHICH YOU ARE PUSHING.
Split pushing is when you go to a side lane when nobody else is around, farm as much as possible and push the lane to the turret. Kill the turret, then recall fast. Run or b immediately you see an enemy approaching, or an enemy leaving another lane and beginning to approach yours. So, if entire enemy team is mid and you are LOSING the game, go bot and kill a turret. Then, b, go top and kill a turret. Go bot and do it again, and again, until you have pushed to the inhibitor. Backdooring also works a bit like this: If entire enemy team is mid, creep round behind and kill the inhib/nexus/nexus turrets while they are not looking. Recall fast if you see someone from their team recalling to come and stop you. Split pushing and backdooring is best done when using Teleport, however you can quite easily do it without.

This section is coming soon! I will update it ASAP.

This is the end of my guide. As I said in the beginning, please do not downvote without trying it out first, this is my first guide ever. If you have anything you would like me to change, add, or remove, or if you have any suggestions or CONSTRUCTIVE criticism (All plain criticism will be ignored) leave a comment and I will do something about it. Jayce is officially my best top laner, and I do use the things included in this guide in ranked matches. My best score when using this guide is 19-7 in a ranked match. A massive thanks to jhoijhoi who helped me to make this guide, you can find her guide to making a guide guide here if you feel like making your own: http://www.mobafire.com/league-of-legends/build/making-a-guide-101506
Thank you for reading!