Cho'Gath is very good at solo top. He's Ability Power and Offtank. If you build him how I build him, it's really difficult to get killed. You've much health and more then 70% reduced physical damage and 30% returns.
This is a small guide that'll help you to play Cho'Gath.
Good Luck!

Pros:

+ Funny to play
+ Very nice abilities
+ Stealing Ult
+ Much hp without items
+ Nice for in teamfights
+ The longest silence in LoL Feral Scream (3s!!)
+ Fast without boots
+ Perfect escapes
+ True damage ult

Cons:

- Fast OOM (Out Of Mana) at the beginning of the game
- You need to learn where placing Rupture
- Much mana cost
- Rupture miss while be ganked is a shame
- Upon dead with 1,3,5 stacks with Feast. It is rounded down (1->0, 3->1 and 5->2)
- Not easy to play, if you play him for the first time. Learn Cho'Gath by playing vs Bots

Runes, why Ability Power?

Ability Power is just the way to kill. All your abilities need ability power. If you have a few ability power at the beginning of the game your abilities deal more damage. And you're an offtank so you don't need health now. And your magic penetration is always nice for better damage.

I play Cho'Gath with 9/21/0. Your Ability Power is increased and Magic penetration too. Cho'Gath is an offtank (or "tank") and need tank-masteries. That's always good and better for later (and for in teamfights).

Items are important. I play him with much hp. Warmog's Armor is THE way to get health and health regen. But a few Ability Power isn't bad, health and mana regen too.
Cho'Gath is trying to get all the damage. Stun, taunt etc. You need hp and Tenacity to survive it.
Frozen Heart decreased CD (= Cooldown). That's better if you're dead and need full stacks (= max 900hp). Guardian Angel gives you a new 'chance' for the last kills, but it's 5 MINUTES CD!. Thornmail is always nice if you play vs much ad(carries). And Rylai's Crystal Scepter for more slow, ability power and health. (NOTE: For Rylai's Crystal Scepter you need to remove Doran's Ring. You can also remove it earlier in game).

Items are important. I play him with much hp. Warmog's Armor is THE way to get health and health regen. But a few Ability Power isn't bad, health and mana regen too.
Cho'Gath is trying to get all the damage. Stun, taunt etc. You need hp and Tenacity to survive it.
Frozen Heart decreased CD (= Cooldown). That's better if you're dead and need full stacks (= max 900hp). Guardian Angel gives you a new 'chance' for the last kills, but it's 5 MINUTES CD!. Force of Nature will give you more magic resistance and incresed your health regen (and give bonus hp regen for all your hp)! Movement speed is very good for tank, you can go fast IN teamfight (or with Teleport) and you can go fast OUT teamfight. Spirit Visage is very good too increase your health regen more then enough. (NOTE: For Spirit Visage you need to remove Doran's Ring. You can also remove it earlier in game). (You've 10% more cd!)

Cho'Gath passive is:
Carnivore Whenever Cho'Gath kills a unit, he recovers 34 - 68 (+2.0 each level) health and 3.5 - 7.75 mana (+0.25 each level).
Cho'Gath Abilities:
Rupture Ruptures the ground at target location. After a delay of 0.65 -> NOW: 0,5! seconds, enemies are launched into the air for 1 second, dealt 80 / 135 / 190 / 245 / 305 (+100% of ability power) magic damage, and have their movement speed slowed by 60% for 3 seconds.
I upgrade it on lvl 1,8,10,12 and 13
Feral Scream Cho'Gath screams in a cone in front of him, silencing enemies for 2 / 2.25 / 2.5 / 2.75 / 3 seconds and causing 75 / 125 / 175 / 225 / 275 (+70% of ability power) magic damage.
I upgrade it on lvl 2,4,5,7 and 9
Vorpal Spikes Toggle: Cho'gath's physical attacks launch spikes which deal 20 / 35 / 50 / 65 / 80 (+30% of ability power) magic damage in a line in front of him.
I upgrade in on lvl 3,14,15,17 and 18
Cho'Gath ULTIMATE:
Feast Devours an enemy unit, dealing 300 / 475 / 650 (+0.7 per ability power) true damage (1,000 to minions) that ignores armor and magic resist. If the target is killed, Cho'Gath grows, gaining 90 / 120 / 150 extra health (max 6 stacks). Cho'Gath loses half his stacks (rounded down) upon death.

Why so?
Rupture It's nice for at the beginning. If you play with 2 on top, you can have first blood with it. You place it and attack it, they're slowed.
Feral Scream The longer the silence, the better it is. The enemies can't run ago. First you use your rupture (knocked airbone + after that slow), then you use Feral Scream and they can't use summoner spells and they're running slow.
Vorpal Spikes You're Ability Power, but more minions in 1 time is always nice. It's "a bonus"
Feast Upgrade your ultimate so fast as you can. That's 6, 11 and 16

I use:
Flash is nice for escapes and if you're going dead (and Guardian Angel is CD or you haven't a Guardian Angel).
Teleport is nice for incomming teamfights, BUT you can do that only ones in the 5 MINUTES.

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You can also use:

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Ignite For hp/regen reduction and last true damage.
Exhaust For fast ganks.
Heal For healing team for last chance.
Smite For JUNGLE-BUILD This isn't jungle build (I don't have jungle-build, perhaps with a new update)
Clairity It's good at the beginning of the game or in teamfight when everybody is low mana.
Promote If you go back for healing you can use promote for incomming gold or starting minion storm.

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DON'T USE THIS:

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Ghost If Rupture miss is why using ghost? You're fast, and Flash is better then!
Revive Why use revive? Guardian Angel is just enough? Or do you need 5 more lifes?
Surge U don't need more AP/Ad spd, you miss then a few AP/AD spd, not important!
Cleanse Noob spell.
Clairvoyance Cho'Gath isn't a support!

At level 6 try to farm with your ult on the biggest minion. It dealt 1000 TRUE damage so there's no "armor" for.
And your ult (= Feast) dealt TRUE damage too enemy champions! (Don't forget your ult!).
If you've 50% hp, go farm and last hit Carnivore

If you'll kill an enemy go place your Rupture and walk back, if there's an enemy in go use your Feral Scream for silencing him! (no escape more!) and kill him at least with your ult (= Feast size) for more hp (if you don't have 6 stacks). The more stacks you earn, the bigger you'll be!
You're faster if you're above 70% hp so SLOW, SLOW, SLOW!!
If you've 50% hp, go farm and last hit Carnivore (passive)
If you play Cho'Gath for the first time, play vs bots. You can learn the basic of Cho'Gath and his abilities

Cho'Gath is buffed:

Rupture Will pull all enemies in the air, but now faster! So you place and it's easier too pull enemies in the air! (from 0.65 -> 0,5s!) BUFF!

Vorpal Spikes// Feast How more stacks with ult, the longer distance the Vorpal Spikes will deal!
For all AD Cho'Gath this is a double buff! If you have ult in the jungle with a few stacks, you have more AoE dmg! And in teamfights you hit more enemies! BUFF!

I hope this'll help you to play Cho'Gath as a pro.
I try to make it bigger, larget etc. I hope you learn Cho'Gath playing as a good man and PM me if you've a nice score!

Here you can see the Positive and negative updates of this Cho'Gath build
* 06-09-'12 New: 'Updates'
* 06-09-'12 New option instead of Thornmail and Rylai's Crystal Scepter (1st build) -> Force of Nature and Spirit Visage (2nd build)
* 06-09-'12 New title: Cho'gath the Solo-Topper or Tank
* 06-09-'12 New: 'Pros / Cons'
* 06-09-'12 Incresed Summoner Spells
* 06-09-'12 Now item taggs and not only the name of it (example: Thornmail -> Thornmail)
* 06-10-'12 New: Skill Sequence
* 08-15-'12 New: Update explain: CHO'GATH IS BUFFED!