Because i'm getting disgusted at how most people play this guy i'm going to go over in my opinion the best way to play Cho'Gath.

First Reminder: Cho'gath IS NOT A TANK!

Abilities:
(Passive) Carnivore: This passive allows Cho'gath to stay in the lane at the start as long as he feels like, the only reason you should go back is to shop.

Rupture: Cho'gath's main damaging ability, now this gives you a nice AOE disable with a 1 AP ratio allowing it to also nuke the opponents health, it can also harras enemy champions at the start as well as farming whole creep waves, i would only recommend beginning to use this at level 3 otherwise the mana cost is too high for the amount of mana you start with.

Feral Scream: Slightly less damaging then Rupture but containing an up to 4 second silence. Again this is the ability i would choose for level 13 since i find it is not worth getting early on since you have enough disable from Rupture.

Vorpal Spikes: The terrible AP ratio on this is simple because it enhances your auto attack making Cho'Gath able to dish out insane damage without even using another ability, this also helps you to farm creep waves and harras enemy champions trying to farm.

Feast: Cho'gath's glorious ultimate, having a ratio of 0.5 with AP Cho'gath can nuke and kill any champion under 800 health at max rank not including extra damage from AP, this can be used on fleeing champions if you are in range for less than a second which shouldn't be hard seeing how big you should be, this is also your finisher for any fight, when they're low enough feast and walk off.

Second Reminder: Cho'gath IS NOT A TANK!

Ability Ordering While Levelling:
Rupture
Vorpal Spikes
Rupture
Vorpal Spikes
Rupture
Feast
Rupture
Vorpal Spikes
Rupture
Vorpal Spikes
Feast
Vorpal Spikes
Feral Scream
Feral Scream
Feral Scream
Feral Scream
Feral Scream

Item Build (End Game):
Sorceror's Boots: The only boots any caster needs with the magic penetration bonus allowing for bigger nukes and harrasment.

Archangels Staff: Getting this early on allows you to begin gaining Mana from the amount of times you can spam Rupture on opponents, giving you a nice ap bonus in the end.

Rylai's Crystal Scepter: Even though Cho'gath doesn't need it, it gives him extra survivability and extra slows on his abilities as well as adding to his AP Pool.

Zhonya's Ring: It's a Zhonya's of course you want it on a caster for the AP.

Lichbane: THe added movement speed should allow for extra feasting and of course extra AP and the bonus to your Damage it all stacks up.

Deathfire's Grasp/Abyssal Scepter: The last item is just a matter of preference, depending on who is on the other team, what day it is, how your feeling that second. Since neither of them grant much extra bonus except the AP.

Basic Strategy:
First Items: Doran's Ring + Health Pot After this you should never ever ever ever ever buy a pot, if you do its a waste.

Rupture: Use from level 3 onwards, trying to spam this at level one is pointless since it does pitiful damage at a high mana cost, but once its level 5 you can kill champions easily with this, a guy is running away just place rupture in front of him and pow he's dead, you should initiate some fights by placing rupture where you think the opponents will go, which normally is backwards even if it's in front of them etc etc. Just kill them

Feral Scream: the second part of the 3 hit combo. just spam it.

Vorpal Spikes: No talking nessessary

3 Hit Combo: Once you have enough AP you should be able to Rupture, Feral Scream and then Feast someone to death.

Runes: Whatever you Fancy

Masteries: Classic 9/0/21 for casters

Spells: Clarity for the extra mana buff and Flash to close the gap to Nom nom nom nom

Hope you enjoy my Chogath Guide and Nom those idiots
Comment and Rate Please.
Last Reminder: CHOGATH IS NOT A TANK!!!