Hi there.
I'm GreenPikachu and this is my first lol build.
I took Cho'Gath because he was the first champion I bought and he's my main champ for premade battles. (And of course because i love to feast smaller champs like Teemo)
The build is focused on playing solo top but also works fine on other lanes.

For the runes I chose flat armor and magic resistance for an early game advantage.
Ability power quintessences and magic penetration reds to keep pressure on my opponent.

Not much to say, I prefer very offensive masterie trees to be an even bigger threat.

Rupture first, perfect for keeping the enemy away and for last hittig some mimions.
Feral scream comes next to kill minion waves even faster and to harass the enemy top laner.
After that I go for vorpal spikes three times. - They do damage, they hit everything in their way and they help a lot in farming.
Obviously feast is improved everytime it's possible.
After that: W < Q < E

The Terror of the Void - It's big, it does damage and you can't defeat it.
Those are the things that come to mind, when you see Cho'Gath.
I start with Doran's Ring to stay in lane as long as possible.
Your passive should get you enough health to disclaim Regrowth Pendant.
On your first visit back to base you should buy Boots of Speed, Null-Magic Mantle (or go straight for Mercury's) and if possible take Glacial Shroud with you.
Those are the main items to keep you alive in early-mid game.
After that rush for Ability Power, buy Abyssal Scepter and Rylai's Crystal Scepter.
You can buy another Null-Magic Mantle, if the enemy team has lots of mages.
You should finish Frozen Heart after that to be more helpful in teamfights and for a big defense boost.
Lich Bane and Rabadon's Deathcap for getting very strong and scary if the game won't come to an end.

I hope you enjoyed the guide, please don't be to harsh with me concerning the layout.
Please leave some comments, ratings and suggestions for improvement.