Hi!
This is my first build here on mobafire so go easy on me :)
First of all, you do not need to follow every little thing in my build, like the runes, I just picked they major AP tank runes.
Your items are important but you can also make some adjustments:

Ruby Crystal => Gives you enough starting HP & your passive will give you HP regen enough

After Ruby Crystal, you can do 2 things:
Warmogs Armor OR Leviathan:
Feeling confident & stronger than your Opponent? Go for Leviathan, its passive will be pretty nice throughout the whole game, but you need to make sure you'll get 20 STACKS!
I do NOT recommend this in a ranked.

Otherwise just go for a warmogs if you got the feeling there won't be alot of kills.
On to the next one, your boots:
Pretty basic, Lots of Mages OR stuns, taunts, fears, ... in the other team => Mercury Threads (will be most of the time)
Lots of AD => Ninja Tabi
NOTE: Some people take the recommended boots; Boots of Swiftness. Do as you like, but I'm not a big fan of it.

Next one is pretty obvious: Guardian Angel
Gives armor + MR + gives revive wich is good for ur FEAST stacks (& Leviathan if you picked that)

--- If you farmed pretty well, you will have all of these items by midgame, you can start tanking now ---

The next item depends on the opposing team:
They got a lot of AD or a fed AD champ? => Go for Thornmail
They got a lot of AP dmg, or a fed mage? => Go for Force Of Nature
They have a balanced team? => Go for Force Of Nature!
Why? The passive is OP for high HP champs like Cho'Gath

=> Get the item you didn't get first, after

-- From here on, you have become immortal IMO --

Last item: Go whatever you wanna go with.
Your team doesn't do enough DMG? Help them, by buying an AP item.
The other team got armor or magic penetration? Get some more armor or MR
___________________________________________________

Try always to take a solo lane with Cho.
FARMING is the key to a strong Cho'Gath .. FARM FARM FARM
I try getting 6 feast stacks ASAP!
Thats actually it, farm in early game => untill you got a GUARDIAN


You got any questions/comments? You think I'm wrong about something?
Please post them! I'm curious what you guys think of this build! I'm not the best player in the world, so I can have made some mistakes!