Greetings, this is my very first guide in mobafire, trying to keep it simple and plain.
I have been playing support since early season 2, but i really found Janna to be something of a miracle in ranked games, so i felt like sharing my experience in this particular champion with you guys.

If experience has tought me something since i started playing Janna, it is that this lady is all about 3 things: Protection, Mobility, Utility. While others find her early lane phase hard and somewhat weak, compared to other more offensive support characters like Thresh, Leona or Annie, I think that as the game goes by, the stronger Janna gets in both lane and teamplay.

Pros +Great Protection of teammates +High movement speed +Respectable CC when it comes to offense (strong slow, Knockup) +Strong AoE heal if channeled properly +Shields away tons of damage +Great AD boost to teammates with Eye of the Storm

Cons -Weak early game(long cooldowns, no stuns, low damage) -Little damage-dealing capabilities compared to other supports -Slows one single enemy at the expense of own speed -Hard to master

With the changes that season 4 brought, I choose a 9-0-21 mastery page for Janna, placing the 9 points in offense tree, since her abilities now stack greatly with AP, and then i go all the way to the Utility tree, 'cause as i said earlier, Janna is all about utility.

Offensive Tree
Sorcery Must-have mastery for every champion that prioritises his own spells (and not auto-attacks) as a way of playing.

Mental Force AP per level here, helps more and more as time goes by. Just like Janna does, in a way!

Expose Weakness Slightly increases your team's damage upon your target. Not bad, really!

Arcane Mastery Plain-old AP boost, helps you all the way from 1st to last minute!
Defensive Tree
I choose to use none of the mastery points here, since most(if not all) of the first masteries of this tree do not give you much of an advantage in early-game, in which Janna is in most need. I prefer using runes for that job(health, armor), but we will get to that. So, no defensive runes!
Utility Tree
I strongly believe that during this season, this utility tree is now the support's bread n' butter. Lots of stuff here, helping your laning and whole game experience.

Fleet of Foot Minor monement speed boost. You will need movement speed when supporting, warding areas faster, getting to allies in less time, etc. It's all about speed,people!

Meditation Janna is a mage-support, which means a need for mana. Extra mana regen will help you throughout the whole game, so this is a must-have.

Summoner's Insight Just something to get you to the next masteries below these. Having slightly lower cooldown on your flash doesn't hurt either.


Alchemist You will find yourself using potions, mostly during the laning phase. This thingy here shall enhance them a little, and make your gold well spent!

Greed Gold/sec bonus. You're a support, so nothing more to say here.

Culinary Master Converts your health pots into cookies. An instant replenish of health and mana every time you use them. Good enough for the lane. Give this man a cookie!

Scavenger Stacks greatly with the Ancient Coin item, and the ones made out of it. Very profitable!

Wealth Sends you at the lane with an extra potion. Who can say no to that!

Inspiration Gives you a small xp bonus if you are left behind the team. Might help more than you realise, especially in mid to late game.

Bandit or Pickpoket if you prefer. One hit once in a while goes a long way!

Intelligence Intelligence is power, people! Along with the Sorcery you start off with a 10% CDR from level 1. What else is there to ask!

Wanderer Your own personal Mobility Boots, and it comes to you for free. Great mastery, movement speed is what we're after of course!

At this point, there's a good variety of Runes you can choose for supports. Some people prefer pure offense, going all the way with some Greater Mark of Magic Penetration , along with Greater Glyph of Ability Power and Greater Quintessence of Ability Power .
Not my style, but i won't say it is wrong. Everyone can play efficiently the way they want, within the reason, of course.

Seals

Greater Seal of Armor This is an absolut must. Some people choose gold/sec in Seals, or perhaps CDR. I disagree. You need this armor, and when i say "need", i mean like "take them or get poked to hell and back again" from the enemy carry. You get about 13 bonus armor from these alone, and trust me, they do make a difference during the first levels.

Marks

Greater Mark of Armor I choose these instead of anything else for extra armor. As I mentioned before, Janna needs help during the early levels. Something to make you useful, and help you survive enemy poke and gangs.

Glyphs

Greater Glyph of Scaling Magic Resist You need those a lot if you are up against an enemy AP-based support, like Annie or Thresh. They also help a great deal in teamfights, since you are a squishy target and the enemy team will focus you from times to times. Alternative options are Greater Glyph of Ability Power . providing you with more power for your shield and heal, as well improving your damage. Both are great choices, depending on the enemy picks.

Quintessences

Greater Quintessence of Health I use these, once again for better early game. I know a lot would find this choice strange, if anything. Greater Quintessence of Ability Power is also a very good choice once again. It is really a matter of style here, since both choices are useful and viable.

My two classic all-time choices are:

Propably the most Overpowered, all-time saver summoner spell of all. Good for escaping, for chasing others, for positioning, for getting you out of a hard situation. Everyone needs flash!

Exhaust is great for weakening an opponent for 2.5 seconds, especially if he uses AD. Casting it on enemy ad carry or assassin/AD bruiser can really make the difference in a fight.

Other Summoner Spells that come in handy, according to me:

Ignite Deals true damage over time, plus reduced healing for its duration. It would be useful should the enemy team have a strong healing ability, like soraka or mundo. Not a bad choice either, especially against AP-based teams.

As for the remaining spells, i just find tottaly unsuitable for support, so i choose not to mention them.

Max Eye of the Storm first. This is basically your most common spell, and Janna's signature skill. Gives a good amount of shield, plus bonus AD while the shield stands, and both shield and AD bonus scale with your AP. When you reach the core build, the cooldown of this spell is 6 seconds. This is a great spell for both your team's offense and protection.

Max Zephyr second. Permanent, strong movement speed buff (while it's not used and on cooldown ofc) plus a powerful single-target slow spell. When it's all maxed out, Janna with be like a racing car on the map. Moreover, the slow is one of the most powerful ones in LoL.

Max Howling Gale last. No need to have it more than level 1 for the most part of the game, as only the damage incrases when you raise its level.

Finally, you should raise Monsoon at levels 6,11 and 16. Its cooldown goes down as you do, and heal increases. It's a really worthy ability.

First things first, there should never be a standard 6-item build on any champion. Every seasoned player knows that adaptation to one's team as well as the enemy's is the key in every game when it comes to items. One again, THERE IS NO STANDARD ALL-TIME BUILD!

Now, I do believe in certain standards when it comes to Janna, and support champions in general.
We have Sightstone. This item is essential. No matter your team, no matter your performance in a game and no matter pretty much anything else, this is your leading item as a support. Your teammates need wards on the map at all times. Vision is a great advantage to have. Controling your objectives is vital. And the bonus health it offers doesn't hurt either!
Afterwards, there is Talisman of Ascension. Now, this is your all-time money-maker as well as your primary utility tool when it comes to Janna. I would not replace it for any other support item (And I will explain below why).

All in all, my build is supposed to help, to "support" in every way possible the teammates, while being able to sustain a good amount of damage. When it comes to an AD-based enemy team, i will go with a Frozen Heart, and maybe even Ninja Tabi or Randuin's Omen. Then again, if they are AP-based then there is Locket of the Iron Solari, Banshee's Veil and Twin Shadows, the later one being very handy in many situations. I do not rely on any damaging capabilities, i leave that to my team when i choose to pick Janna.

For starters:

Ancient Coin+ Vision Ward+ Health Potion+ Mana Potion This is my standard build.

Ancient Coin is a great money maker, combined with Scavenger . It does not make you push the lane, in addition to Relic Shield and it does not force you into riskful auto-hits like the Spellthief's Edge requires.

A Stealth Ward, combined with your carry's trinket should be enough warding for the first few minutes. On yor first back, you should definitely buy at least 2 more wards, or should the lane go very well, just rush for Sightstone.

I prefer to take one of each potions, instead of 2x Health Potion, which turn into cookies. I understand that Ancient Coin along with your masteries should provide enough Mana Regen to get you going, but it never hurts to be prepared. Consider your Mana Potion a safety measure, just in case.

On your first back, you should rash for Nomad's Medallion, and then straight for Sightstone. You do not have to make it a Ruby Sightstone at once, it is not that essential during the laning phase, in case you want to invest the rest of your gold elsewhere.

After these two, definitely finish your Talisman of Ascension. It will lower your abilities cooldowns drastically (one of the few items now with 20% CDR), plus the sweet active ability which can save you from a gang, or other situation. Plus, it can help your carry chase down injured enemies.

Make a Chalice of Harmony if you find yourself in need of more regen, which i usually do not, myself. Nevertheless, you should really rush for Mikael's Blessing if the enemy team posseses powerful Crowd Control abilites, like Terrify or Enchanted Crystal Arrow. It can really make a huge difference there.

Finally, for trinkets. I strongly suggest you start with a Stealth Ward, for the extra ward in early laning phase. Once you complete Ruby Sightstone, you should immediately replace it with Sweeping Lens, since you cannot place more than 3 wards at a time. Try to not stall upgrading your Lens into Greater Lens for too much, i actually suggest you do that after your core build is complete. Destroying the enemy's vision is a great way to fight them!

Boots:

Mobility Boots This is a great choice, since it will help you reach any place on the map in zero time, combined with Wanderer and Zephyr, plus your passive Tailwind.

Mercury's Treads This is a must-have item, if your enemy team posseses a good amount of Crowd Control. Heavy Crowd Control can detain you from enabling your ability combos, making you tottaly unable to save your teammates when the time comes. Plus, extra magic resist.

Ninja Tabi This is a very underrated item by most players. If you find yourself focused a lot by an AD-based enemy team, then this will be a good choice. Remember, supporting your team is what you strive for, but you need to survive in order to do that! Do not be afraid to get all tanky, if the game calls for it!

Enchantmnents

Enchantment: Captain Expensive, and yet this enchantment comes in real handy. Good for late-game pushing, plus leading the chase against injured enemies, or helping your team get out of a lost fight just by running towards you.

Enchantment: Alacrity Cheap, and useful. This enchantment can be bought in even earlier stages of the game, it offers you a slight boost in movement speed. It might make a difference, you never know.

Enchantment: Homeguard Great, especially when the game makes you play difensively. It helps you travel a distance from your base a lot. Consider it if you find yourself defending inner turrets while the enemies push every lane a lot.

Defensive Items

Frozen Heart Against AD, this must be your first choice. Great armor boost, plus 20% CDR whcih is what you need most on Janna.

Ardent Censer Stacks great with Janna's Eye of the Storm , plus the mana regen and movement speed are a considerable boost. I recommend it, hands down!

Locket of the Iron Solari An almost-core item for you. Provided a sweeeeet AoE shield for all of your team, plus 10% CDR for you, health, armor and 20 magic resist aura for your team. If the enemies have a strong AP champion, this is a must-have item.

Banshee's Veil Buy this one if you find yourself focused by a lot of spells, especially strong single-target abilities like Primordial Burst, or even AoE ones like Explosive Cask. In general, it is good versus an AP-based team.

Randuin's Omen I only recommend this item if you find yourself against very strong AD-based opponents, where you need to step up and protect your carry for harm in more ways than your shield and CC.

Thornmail Only make this item in late game if you need to sustain great, i mean pretty great Physical damage. It can really make the process of killing you a nightmare for an assassin or a carry, if you combine it with other defensive items.

Offensive Items

Twin Shadows An excellent choice, if you are looking for some more CC with your ghosts searching out and clinging upon hidden enemies, slowing them. Plus, more magic resist and a small boost of AP for you. A very good almost-core item for janna and almost all AP-based supports.

Deathfire Grasp RIP

Alternative Items

Zeke's Herald More CDR for you (in case you did not reach 40% yet), plus an AD+Lifesteal bonus for your team. Can prove pretty useful, if your team is full of AD Bruisers.

Rabadon's Deathcap I would only buy this item as my last one, or perhaps semi-final. It will certainly give you a great AP boost, making all your abilities much more powerful. If power is what you are asking for, Deathcap is what you need!

Mejai's Soulstealer Hands down, this item is a gamble. If the early game goes well, and you manage to outscale your opponents in bot late due to succesful gangs or excessive outplaying, then this might prove a superpower of an item. You only need to be evasive, and get tons of assists.

Howling Gale Use this one to knock enemies up in the air. Remember, the more you let it charge, the more time they're up in the air. Extremely handy against opponents that run towards you and your carry, like Volibear or Singed. Do not hesitate to let it go early on, better not let them know its coming, especially during a chase.

Zephyr Enable its active effect only when you really need to stop an enemy's movement. Consider that slowing Janna down can make her an easy target.

Eye of the Storm Use it in a fight to save a life, or to end a life. Target your tank to make him take on more damage, but you can also target your ranged carry, even if he is untouched by enemies, and boost his damage greatly. Remember, this is much more than just a shield.

Monsoon Now, this spell is basically what makes a good Janna player, and a bad one. Its primary use is to be a combo-breaker. Seperate enemies from each other. A major counter for AoE stuns and snares like Curse of the Sad Mummy, or Solar Flare. Careful though, it can also lead enemies out of your team's own combo, if Amumu or Leona (for example) are on your side.

If you can hold your ground, let it channel and you watch your teammate's health bars fuel during the time. The healing is pretty powerful, so don't be hasty to interrupt it.

Finally, you can position yourself in front of the enemy and knock them towards your teammates position, making them vulnerable. This is its offensive capability, and it's quite good too!

Janna is a champion which is easy to learn, but hard to master. She needs patience, but if you make it through the early game she can protect her AD carry from anything. Her ulti is a real fight-changer if used properly, and so are her other abilities.

Thank you for reading my first guide, i hope it could be of help. I will update this guide over time, and give more tips and strategies over the art of supporting with Janna. I will be glad to see any votes or criticisms about my work. Thank you.