To give you the basic low-down on Fizz. Fizz is an Magic-Based Melee Assassin. In my opinion hes almost like the melee combination of Veigar, Tristana, and Graves. This is because not only do his spells give you heavy nuking abilities like Veigar but they also give you the ability to jump in an out of combat on a whim. This makes tower diving incredibly easy and entirely possible by Lvl 3. However with all of the Pro's for a decently balanced champion, there are always the cons. His early game is very weak which is why the runes and masteries shown above are created to give you the biggest advantage early on before you can get RoA.

In the Early Game, Fizz is incredibly weak, but with smart playing you can get a kill in before the enemies even start battling it out. You want to start off with Boots and three Health Potion. Have yourself ready to go so that you can purchase these and hide out in the bushes in your lane and prepare to strike the weaker of the enemy champions. If done in coordination with your teammate and a double ignite, guarantees a First Blood.
Once you start battling the minions, I've discovered with Fizz you may let your teammate get a greater chunk of the minions, this is fine if you're Laning with Veigar or Nasus (although if your smart you should be laning with a Ranged DPS), which means if you are not killing enemy champions you will have NO MONEY. Once you get to Level 3 it comes down to is activating Seastone Trident then imediately speed in with Urchin Strike and bounce back out with Playful / Trickster. This will effectively harass the enemy champions enough to when you get to level 6. When you go back to get items or if you somehow die, I suggest picking up an equal number of Health Potion, especially if your new to Fizz since it is crucial to have enough mana to use your Ult right when you reach level 6.


Keep in mind that you should go back to get Catalyst the Protector and Ionian Boots of Lucidity once you get a chance, since they will be necessary to have enough mana and low enough cool-down to continually use Chum the Waters in the Mid Game.

At this point of the game, you should already have your Boots and at least the Catalyst (if not then Rod of Ages). If you haven't gotten RoA, that should be your top priority since it will give you enough health to escape a gank, enough mana to keep using Chum the Waters, and enough AP to do some serious damage within a short time frame. At the sametime you should be ganking up a storm, if I am in the top lane I'll follow the pattern: gank top lane, recall back and buy two Sight Wards/items, get blue buff, gank bottom lane, place ward in between bot&mid , gank mid lane, place ward in between mid&top, farm and/or gank top lane and then rinse and repeat when the wards are about to run out.

Note: when buying items make SURE you buy TWO wards and place them, if you are methodical about this, it will stop lane switching and save your *** while switching lanes back and forth to gank. Also consult the buying order of items below to receive explanations for the purchase order.

This is the part of the game where Fizz really gets to shine, if you haven't been doing well with your k/d/a ratio, I can guarantee you will start doing better now. At this point, our team and the other team are finally starting to get together and push lanes as a team. Now is when you should have at least 7-15 kills and Void Staff & Rylai's Crystal Scepter depending on how well you have been doing.
As Fizz if your team seems hesitant and the tank is wussing out, you can start the gank by distracting the enemy team with a [W]+[Q]+[E] combo, in which you should be using Playful / Trickster to jump back towards where your team is hiding/waiting. This will fire off the starting pistol and get everybody fighting each other. After you wait for Seastone Trident & Urchin Strike to refresh you hit most of then enemy champions with Chum the Waters hit a with a [W]+[Q] combo before the shark attacks, and will most likely result in a kill.

Explanations for the Main Build:



I consider the Boots of Lucidity to be necessary, because Chum the Waters unlike what Mobafire says, starts off with an 100 sec cooldown and costs 150 mana, this makes it necessary to keep using it as the enemy keeps respawning or to use it multiple times in a longer team battle. Not to mention it will reduce the time of Playful / Trickster

Substitutes

Sorcerer's Shoes is a clear possibility if you would prefer not to use Void Staff, since if the other team isn't catching on you can do absurd amounts of magic damage, Sorcerer's Shoes will have sufficient enough Magic Penetration to get through a normal champion.


This one is basically self-explanatory. If the other team is crazy with the amounts of stuns or disables or silences, Tenacity is a must.



Rod of Ages is absolutely mandatory for Fizz. Period. As I explained early it gives Health, Mana, and AP, which are all crucial for your Middle Game sustainability, since it will also recharge Health and Man every time you gain a level it better to get earlier than later.

NO SUBSTITUTES.

Now is where the purchasing order depends more on your preference and the people you are playing.


Void Staff is the Most fluid out of all of the items listed in this build. As I said early if you wanna throw in Sorcerer's Shoes you can definitely trade this out for a different item. That being said I suggest you either use this space to push up the rest of the items one slot and grab an extra Rabadon's Deathcap at the end. However if it looks like one of the enemy champions is gonna get fed by a noob on your team, I STRONGLY SUGGEST you swap out Void Staff for either a Thornmail or Force of Nature, depending on the Enemy Champion in question.

Substitutes

As stated above this is a good choice when you are being rushed to find yourself against a fed AD champion. This is a great way to see the immediate difference when facing such a champion, since with Fizz's skills they should be only to land a few number of attacks. In context of purchasing Thornmail, it has been suggested that you purchase Zhoyna's Hourglass instead. Now this is perfectly acceptable, however Zhonya's is a bit pricer and doesn't have the crucial passive that Thornmail has. I would suggest, since it is a good point, that you purchase Zhonya's in the late game to replace Thornmail, when your dying to get that last bit of AP in and the 3 second stasis can truly become the difference between a win and a loss.


Now a similar note as Thornmail, Force of Nature is not an AP item and Abyssal Mask is and Abyssal Scepter also has significant magic resistance. In all fairness Abyssal Scepter is the smarter direct replacement in which you wouldn't need to substitute a Lich Bane for an additional Rabadon's Deathcap. However, I find myself liking of the fact that it is possible to get the 8% additional movement speed under Force of Nature, and so if you are being pressed for kill and you don't have the money to spend freely I would suggest Abyssal Mask.



This item is another perfect fit for Fizz, and I would strongly stand against substituting it for another item since it gives a LOT of ability power, health, and gives you a slow-on-hit guarantee, which are all CRUCIAL for team battles in the middle-to-end game.


Lich Bane is great with this standard build, since it rounds it out with additional magic resistance, a large amount of ability power, and is one of the few items that grants additional movement speed. The only way I can see it not becoming crucial is if you have subbed out Void Staff for a Force of Nature at which point you could just start on your first Rabadon's Deathcap


Rabadon's Deathcap is a great end of the game clincher since at this point, RoA has definitely Maxed out and the Crystal Scepter gives you another boost in your health, you can begin to focus more on obliterating the competition when it gets close to you and Rabadon's Deathcap


Possible Alternate Builds explained:

I hope you all give some of the things I mentioned a little bit of a thought and I hope it improves your game at least a little bit!
Thank you to Daxner and Rohammers for your help in the making of this build!