Fizz is an AP Burst Assassin. In team fights, he seeks out squishy opponents (preferably AD and AP carries) and brings them from 100% to 20%ish. In lane, especially when he is mid, he has the ability to be really aggressive from early on. In my opinion, Fizz epitomizes aggressive playstyle.

Marks (Red): Magic Penetration
This is an easy choice. All of Fizz's damage (with the exception of his AA and Lich Bane proc) is magical. There are no other decent options.

Seals (Yellow): This is the slot with the most options. There are 4 good ones: HP/Lvl, AP/Lvl, Armor, and Mana Regen/5. HP/Lvl is better against APs and is more effective late game, armor is better against Tanky DPSers, and Mana Regen/5 is for when your masteries don't provide any mana regen and you expect to need to use E a lot. HP/Lvl is better than armor late game because when you do die, it's gonna be either because you got bursted down (which is usually magical damage) or because you got caught by a majority of the enemy team, in which case you would likely die anyway. WW Tabzz likes using AP/Lvl, but I'm not sure why. At level 6 they only give about 6 AP, and at level 18 they only give about 20 AP (when taking masteries and Rabadon into consideration). Seems to me like the HP is more useful.

Glyphs (Blue): Scaling AP or, situationally, Flat MR. Fizz has great ratios, it's a shame not to use them. MPen from glyphs is so low that the scaling AP runes actually deal more damage late game, and early game the difference is negligable. Flat MR is only for lanes where there is a high chance you might get bursted down (le blanc), or against Tanky DPS who have a form of magic damage/won't get bursted down, such as udyr . Tabzz uses AP/Lvl, Ocelote and MoMa use Flat MR. I use AP/Lvl.

Ignite adds a lot of damage. For a burst champion like Fizz, this can give you the extra damage to kill off an opponent. Combined with W's passive and active, this creates stressful situations for your opponent where his health is just dropping even though he's disengaged and running really, really far away. Ignite can give you the early advantage and help you snowball.

Flash is a universal summoner spell for just about everyone, Fizz included. Flash is honestly better than Ghost in most regards except for maybe the cooldown. Combined with his E, you have two ways to get over walls, plus flash can lead to 'surprise' initiates, whereas ghost can never be a surprise. I've killed a lot of people who were low with flash into QW for the kill. I could never do this with ghost. However, due to the fact Fizz's E is effectively a free flash, this summoner spell is not mandatory and summoner spells that increase his trading/dueling/fighting potential can easily take it's place.

Exhaust is really awesome if you want to be super aggressive. Combined with ignite it makes you a killing machine and very difficult to duel, but on the flip-side doesn't give you as much survivability. If you take the summoner mastery on the offence tree their magic resistance is reduced by 10, which is a nice boost to damage. Taking exhaust will allow you to stick to nearly any target you want.

The masteries are basic ap masteries, most of the ap champs you see will be using this exact mastery. and it's no different for fizz.

Unless you really counter your opponent to the point where he's turret hugging fizz is a hard farmer till mid-late game where he gets his ap up and his can kill all the minions, what i suggest is that you ask for a gank, so you could kill your opponent and then farm so you have the upper hand, also you want to buy sight ward so you can know where your enemy is, and where your enemy jungle is! WARDS WARDS WARDS WARDS WARDS!!!

This guide is not yet finished, I will be adding more onto it later, but for now, this is it, I hope you enjoy it! Go kill some noobs!