I first made this build on LoL King and Copy Pasted it. You might see some [spell].png (example fizzjump.png) in the text. I was too lazy to remove them so I did not removed them.

PASSIVE --- INNATE: Fizz is perpetually Ghost ghosted and takes 4 - 14 (based on level) less physical damagefrom basic attacks.

The way the damage reduction is applied gives fizz a huge advantage. When you receive damage from a basic attack, the first reduction will be the resistance of Fizz (should be around 60% to 80% at the late game). After the physical reduction is calculated, this is the time fizz passive proc. This gives Fizz passive a very good advantage, specially if you face champions with low basic attack damage (like Kalista or tanks).


Pros:

Hard to kill;

Lane bully;

Threaten carries of the other team;

Good kill potential all the game (even if he does not assassinate someone);

Hybrid Damage ( Hard to build against).

Cons:

Lack of certain AoE CC;

Not as tanky as pure tank champions;

Gets useless if you fall too behind ( although you could build out AP and try to come back).

For the runes, the seals I use are 9 flat +1 armor (9 armor). Seals are the best runes for armor resistance giving us the biggest resistance possible. If you want, you can change your flat armor for armor per level seals if you face low physical threat lane opponent (for example: Nautilus, Rammus, Gragas, etc.). This will give us 0.16 armor per level ( 1.44 armor / level, 25.92 armor @ level 18). If you face physical lane bully (example : Renekton, Graves, Quinn, etc.) the flat armor will be better in the early minutes of the game.

The marks I use are the following : 5 flat +0.91 armor (4.55 armor) and 4 +0.13 (0.52 attack/ level, 9,36 attack @ level 18). Since Fizz have a decent base of attack damage (58 attack + 3 attack / level) and we will take the +5 damage to minions/monsters, we do not have any problem at farming under the tower. We also have our W ( Seastone Trident - fizzseastonepassive.png ) to help us out to clear some minions. If you face a team with only magic damage, change the armor for flat magic resistance (but the chance that this happens is really low).

The best option for glyphs is the +0.16 magic resistance per level runes (1.44 magic resistance / level, 25.92 magic resistance @ level 18). Since magic damage characters in the top lane scale better with levels, we should scale our magic resistance with level. This will give us the best option past level 10.

At least, for our Quintessences, we will use one +2.25 flat physical damage and two +0.25 physical damage per level (+0.5 physical damage/level , 9 attack @ level 18). This will give us the extra damage on our basic attack that we need to finish off what we want to kill.

For the masteries, we will go 0 - 12 - 18.

In the Resolve tree ( 18 ), we will need to finish with Grasp of the Undying ( Every 4 seconds in combat, your next attack against an enemy champion steals life equal to 3% of your max Health).
This keystone is very strong with fizz for few reasons:

His Urchin Strike ( Q - fizzpiercingstrike.png ) can activate the Grasp of the Undying;

When he auto-attack, his Seastone Trident ( W - fizzseastonepassive.png ) will burn the target over 3 seconds. On that 3 second, you are still considered "In-combat". So even if only attack once every 2-4 seconds, grasp timer will still be up. You are playing Fizz tank and Grasp scale with your maximum Health.

To reach the Grasp of the Undying keystone, the best spec option is the following:

Unyielding (+5% Armor / Magic Resistance)
Tough Skin (-2 damage took by basic attack from Monsters/Champions)
Veteran's Scars (+45 Health)
Perseverance (+50% increased base HP regen, +200% increased base Health regen is under 25% of max HP)
Legendary Guardian (+3 Armor / Magic Resistance for each enemy champions)

In the Cunning Tree ( 12 ), this is the best option for spec:

Savagery (+5 damage on Minions / Monsters)
Secret Stash (+10% increased elixir / potions effect. Potions = Cookies)
Merciless ( +5% increased damage on champions with less than 40% max HP)
Bandit (+10 gold on hitting enemy champions with an auto attack ( 5 second cooldown ) )

NOTE: I do take Bandit instead of Dangerous Game since you often do small trades with Fizz with your Grasp. You gain approximately 500 - 1000 gold in 25 minutes if you do not get smashed on.

For the starting items, you have 3 possibility:

Poke or Magical Champions : Start with Corrupting Potion;
Low Damage Threat : Start with Ruby Crystal and 2 Potion;
High Physical Champion : Start with Cloth Armor and 4 Potion.


In the MUST NEED items:

Sunfire Cape: Good Armor, Good HP, very good wave clear aura, very good degeneration aura against champions. This item shines on extended fights (on which tank Fizz is good);
Titanic Hydra: Your Q ( Urchin Strike - fizzpiercingstrike.png ) can proc the titanic passive effect. This adds A LOT of kill potential (specially against squishy target in the late game) and gives an auto attack reset for your W (Seastone Trident - fizzseastonepassive.png ).
Zz' Rot Portal is very good resistance item and pushing item. This give Fizz the ability to be tankier and to be able to split push more effectively. If you want to focus more on team fighting, Zz' Rot Portal may not be the best item for Fizz then but I still highly recommend that item.

Situational Items:
You have enough resistance : Rylai's Crystal Scepter and Rod of Ages gives you more HP and more damage (+100 AP for Rylai's and +120 for Rod of Ages). Those well rounded items give fizz more assassination power while buffing up his tankyness;
You have enough HP : Abyssal Scepter , Iceborn Gauntlet, Maw of Malmortious and Banner of Command gives you some AP to kill targets more effectively while giving your more resistance or anti-death shield (Maw of Malmortious). On the other side, you can take the Banner of Command to split push instead of taking a damage focused item (Reminder : Banner of Command gives some AP also).
You need to be tankier : Iceborn Gauntlet, Dead's Man Plate, Banshee's Veil, Spirit Visage (and much more) gives you the best option for some HP and for resistance. I really like taking Iceborn Gauntlet because fizz can use that item so effectively (specially with his Urchin Strike - fizzpiercingstrike.png ).

For a top lane match up, I recommend taking Flash - Teleport because Teleport gives you a global presence. Fizz have very good follow up after his teleport since he can Chum of the Waters ( R - fizzmarinerdoom.png ) or Playful / Trickster ( E - fizzjump.png ) then Urchin Strike ( Q - fizzpiercingstrike.png ) on the target. This also allows you to return back to lane quickly in case you need to affirm more lane dominance or need to catch up in farm/ XP.

You can also take Flash - Ignite for more lane bully. Since most of the top lanes are running Flash and Teleport, you have a 150+ true damage (and Grievous Wound) more than the opponent. You could use that advantage to snowball the game but you will not be able to do global plays and split pushing in the late game.

Maxing W ( Seastone Trident - fizzseastonepassive.png ) first is the best option since this is the skill that deals the most damage on extended fights. I recommend maxing Q ( Urchin Strike - fizzpiercingstrike.png ) after since this will lower the cooldown on it of -1 second for each level ( starting at 10 seconds at level 1 ). This will increase your chase potential and overall damage since Q ( Urchin Strike - fizzpiercingstrike.png ) also proc W ( Seastone Trident - fizzseastonepassive.png ) and Grasp of the Undying. You could max out E (Playful / Trickster - fizzjump.png ) second if you prefer to play safer or if you need to slow someone.
Take a point in your R (Chum the Waters - fizzmarinerdoom.png on level 6 - 11 - 16 (like every other champions).