Sup? first guide here, hope it goes well :)
Played 5-6 games with fizz so far, and he's super awesome.
Thing is, you have to be pretty careful with him, you can easily become either super-fed or super-dead - by thus feeding the enemy team and ruining the game.

So.. The runes i chose are half durability-aimed and half magicPen-aimed, as i said earlier - you can easily dominate a game, but you have to play carefully in order to so, which is the reason for those armor seals.
The other runes I suggested are for ***KICKING reasons alone, giving you a great burst damage to start with - get your killing early and stay one stay a head of them.

Now, what we have here is a pretty simple mastery tree.
Goal: Get strong - and do so fast while supporting our Sspells and gaining just a little bit more durability for the early stage.
Not rly much to add, just look at it and figure it out.

Here's the fun part :)
As far as it goes for item order - get your rod of ages and boots as fast as you can.
Now - if you had a good start, feel free to keep on going like the build tell you to, But!(and it's a big but :P) if in any stage of the game you feel a little left behind or like you can't hold on in fight, get warmog's, the reason i put it last is because i usually do have a great start, and find it unneeded.
If you got to the leach bane and still feel strong enough to go on without warmog's, just get another ap engine :D.

*why did i choose warmog's? because it fits great with fizz's play style of hit and run.
Just jump in, make lots of damage, get out and regen quickly, you can choose to get banshees or any other armor you'd like to, depending on how well you are doing.

I like to get urchins strike and then playful first. why? cus it's a great freakin' harass tool.
The skill sequence I used is very dynamic - like i believe it should be with such a dynamic champion.
Fizz is very team dependent - therefor you should start chainging skill order when mid-game arrives(when ppl around you start to gank) and you can see how well does your team play together and how do they play(Big teamfight mess where everyone ends up dying, or hunting them down one by one).
How to do that? max your urchins strike first and playful first for 1 by 1 hunt's. 2 great gap closer to ensure any kill you want.
If you see your teamfight or more of a huge mess, max your playful and seastone first, for a little less burst, but better surviving skills and long lasting damage.

Flash - a must for fizz. suit's great with he's play style of getting in, killing and getting out.

For my other Sspell I suggest exhaust or ghost.

Exhaust - slows them down for you, helping a lot in early kills - making it a great Sspell.
Ghost - pretty obvious isn't it? run for your life little fizz, run ( or swim :P)