This is my fizz guide that makes me always go positive every time, except if the are hard core counters.
Fizz is a melee Ap assassin, his job is to kill their Adc/or who ever is most important in a team fight then kill the others.

Pros and Cons

Pros
-Good poke early and late game
-Passive helps laining phase
-Can go un-targetable
-Has good CC including hiss ult and his E
-Easy to get feed if played right
-Q applies on hit effect
-Personaly,one of the best late game champs
-Very good burst

Cons:
-Is melee so can get harassed by ranged champions easily in early laining phase.
-Is a bit squishy
-Ultimate's a skill shot (that I think is actually good as you can snipe with it)

-Good passive that blocks basic attack damage that scales of your levelling, also makes you be able to move through units.

-This ablility makes you dash to a target, dealing on hit effects so Lich bane

-Good basic attack damage that applies magic damage and missing health damage over 3 seconds, when activated it get extra damage and reduces their healing amount by 50%, that can be applied with URCHIN STRIKE, so try do a Seastone Trident then Urchin Strike combo to do good damage.

-Makes Fizz move to an area where you move it with your mouse, the first time you do it, you can wait and the area of the damage is bigger and slows. The second time you do it you can move furtherer but the area is smaller and doesn't slow.

-A skill that slows the target, then after a delay, knocks the enemy/'s airborne dealing massive damage and slowing them 1.5 seconds afterwards.

The abilities sequence:

Recommended

Flash
-Helps getting away or chasing, also works well with
Ignite
-Good for getting kills and works well with Seastone Trident

Their are many combos that you can pull off with Fizz, so I will list some of them:

Poke combo for Level 3-6 Plus:
- Seastone Trident when you are going to engage, then Urchin Strike, get off as many auto attacks as you can, after you have then Playful / Tricksterto retreat or go in and get a kill if they're low. This gives a good poke, so I would try get as MANY auto attacks off before you Playful / Trickster, obviously if you get them low, use you Playful / Trickster and Ignite to secure the kill.

Kill Combo #1:
Seastone Trident> Urchin Strike> Chum the Waters+ Playful / Trickster+ Ignite. Ok, so we use our Seastone Trident to do extra damage and reduce their healing by 50%, then use Chum the Waters to slow them, meanwhile you should be auto attacking. If you know they are not going to survive then don't use Playful / Trickster as that is your escape. If they aren't going to die then use your Playful / Trickster and then Ignite if you have to.

Kill Combo #2 (This is for killing enemies who are reliant on life steal or who have over time damage)
Playful / Trickster> Seastone Trident> Urchin Strike> Chum the Waters If Necessary then Ignite and to top it off then Zhonya's Hourglass
What this combo does is burst them and leave them with Seastone Trident that will deal damage over 3 seconds, also with Chum the Waters and Ignite if necessary, This is all going on while you are in Zhonya's Hourglass so they wont be able to do anything to you and even if they can life steal off minions, jungle or your team, because of Seastone Trident their healing will be reduced by 50%.