Start with Seastone Trident (W) and just wait for lvl two and set your Abilitypoint on Urchin Strike.
Now you can harrass your enemy 1-2 times to get a great positioning for you and your jungler to gank (because they often uses flash and their pots) or to let him going back to base.
[ONE OF THIS TWO THINGS MUST HAPPEN FOR YOUR POSITIONING IN CS AND LVL]

On lvl 3 set your Ablilitypoint on Playful/Trickster

Max Seastone Trident (W) first because of Fizz's dot-dmg you can harrass your enemy everytime so they get tons of damage over time [ Put on your Seastone Trident (W), attack with your Urchin Strike (Q) and get away with your Trickster (E), with this combo you can attack under the enemy tower without being hit ]. {DON'T USE UR TRICKSTER (E) TO DMG AN ENEMY!! DO THIS JUST UNDER YOUR OWN TOWER!!}

Max your "W" is also good because your enemy have to use his heal potions. Now you can lvl up and cs much faster and your enemy won't come back far on his lane.

If you can play Fizz like this and you're just in this situation, it's the perfect moment for a gank on top/bot. [At this point you might be lvl 8-11]

In Teamfights you have to set your focus onto your squishy enemy's. Try to use Chum the Waters (R) usefull and damage/hit all enemy's.
Next you have to use your combo's to kill the AP/AD Carry's first. Get one of him and get away with your "E". Try to get the second and away again. (Fiss CD's are very low so you have to be fast!)

Try not to be stunned/slowed/feared/silenced by your enemy's and try not to be attacked and hit by the enemy's Jungler/Top-Laner. Just focus the Carry's and get away with your "E".

Best Combo to harrass an enemy [low lvl]: W->Q->E(away) and again..
Best Combo to kill an enemy: R->W->Q->E(away)

I know that most of you will ask yourself/others or me why no Deathfire Grasp?!
In my meaning Deathfire Grasp is a lil bit useless in season 3. The activation to make dmg over time is a good thing, but Fizz deals tons of dmg if you use his combo's correctly und max your W first.

Who like's Deathfire Grasp can buy it. In most Situations it's better to buy Iceborn Gauntlet or Zhonya's Hourglass.

It's hard to cs with fizz when your enemy is a ranged AP-Carry.
If my guide helps you and you can play Fizz like me, then you have enough time to farm when your enemy champion is going to base for regeneration or buying something.

Otherwise buy Kage's lucky Pick (As I wrote in my guide) to earn additional gold and try to farm under tower and wait for ganks from your jungler / go and gank Top/Bot Lane.

Pros:

- very mobile champion
- hard to gank
- tons of burst dmg
- strong ganker

Cons:

- CC just with Ultimate "R"
- Squshy in late game
- Dead if get slowed/stunned/silenced/feared
- hard to get cs