I originally wrote this when I had just started playing League (roughly two to three years ago).
Cho'gath has fallen out of favor in the current season, but this build should treat you well.

Cho'gath's abilities are a nice combo, even if they do little damage. He is the equivalent of an AP bruiser with his incidental HP if you build mage.
Let's go over his abilities:

Q (The pop-up): Good CC, prevents the enemy from doing anything until they hit the ground, but it goes off after a duration (So you have to predict the enemy's movements fairly well).

W (The silence): A triangle AOE that silences all foes it hits, and deals a fair amount of damage as well. This is what you max.

E (The spikes): AP damage off of your basic attacks. It can be toggled, in case you are under turret and don't want to hit the other champions.

R (The nom nom nom): Deals true damage that scales with AP. Hence the need for AP. If you kill something with it, you get one of six stacks that increase your health. It also deals 1000 damage to non-champions, so it's a nice smite alternative. If you have flash and a ward, it makes for a decent dragon/baron steal.
It also allows for careful jungling (without the smite ability).

This guide is for a general AP offtank build, but that does not confine it to top. Below are modifications if you plan on taking him to another lane:

Jungle: Get jungle items. I haven't gotten to see the current jungle meta since the season turned over, but I do know one thing:
Get. Vorpal. Spikes. First.
This will save your jungle game. You'll want your W next, then your Q, then max W.
Drop exhaust for smite.

Support: Yes, this is a thing. Because of Cho'gath's mana cost, spellthief's won't work out, so just grab a coin and your W. At level 2, get your Q if you are ahead and your E if you are behind. Build plenty of support tank items instead of AP.
Consider heal if your ADC doesn't have it.

Mid: Pretty much the same as top, but you have to turtle your way through it. Getting too reckless in mid can really mess you up.
Get Mobi boots and gank as early as possible. Fast Cho'gath is scary Cho'gath.
Consider Teleport.

Adc:
Just build six Triforces.
And take ignite.