Hi Welcome to my Fizz "Fish Power" Guide,this is my first Guide here on Mobafire, but I tried my best to make it as useful and informative like other guides. Please help me to improve this Guide by comments and Up/Down votes.
It`s a Guide for players who are still new to Fizz.
I hope I will be able to help you in some ways.

Im going 21/9 masteries, because Fizz is a Squishy caster you will need the armor/health to survive
the early game
Offensive Tree
Summoner's Wrath - Since we take Ignite we would want to put 1 point in here for the 5 AP and AD(will greatly help in farming) while it is on cooldown.
Sorcery - It's cooldown reduction. We want it. We need it. As early as possible.
Blast - More Ability Power? Yes please.
Arcane Knowledge - Magic penetration is big help early on.
Havoc - Increases the damage we deal, how good is that? SO GOOD!!
Mental Force - We put three points here since we would want a lot of AP as early as possible.
Spellsword - This coupled up with Seastone Trident we will be able to deal loads of damage even only with just auto attacks, this will also help us in last hitting.
Archmage - it will increase our Ability Power, which means, more Ability Power, the greater the increase.
Executioner - This will help us kill our opponent in lane. We will be able to deal more damage to enemies below 40% health by 6% more. Great right?

Defensive Tree
Durability - As the game progresses, we will be able to benefit more from this.
Hardiness - The armor we get from this coupled up with our passive, we will be able to take reduced damage from auto attacks(from minions, champions and creeps).
Resistance - The Magic Resist will help us survive more in lane since we are against another AP champ anyway.
Veteran's Scars - Admit it, we need the health early on. We need the health to survive those pesky burst champions.