hello!
I've seen alot of builds for cho all of them where mage-tank but none of them were true tanks and i know Cho' gath isint supposed to be a full tank but i've tried many tanks like rammus, maokai, shen etc. but thay just didnt play theyre role well :/ go figure...
With cho gath u MUST play in lane witha powerful burst champion or at least a good dps... i play with my brother and never did i play badly...

I take armor and magic resist for my runes beacause cho gath is pretty weak at the beginning.
I buy either movement glyphs or deth reduction glyphs
Movement if u feel youre too slow to go nom nom nom on ppl and wanna be faster to land your rupture ahead of fleeing enemy's
Death if u h8 w8ting... or if u die alot...

For masteries u wanna minimise the damage u take so put a few on util. (death time) and 1 on summoner's wrath for your ghost and exaust then put the rest on defence ^^

For the itams you want to buy regrowth pendant 1st so you can stay in your lane longer.

2nd one you have enough money for the levitan, GET IT! as fast as you can then kill and kill kill or be... not killed! get as many assists as you can! this item is great just awesome! its really cheap, gives you alot of health and if you play well 15% less damage! ^^ just.. get it and dont die.

3rd get boots either swiftness ot mercury NOT BOTH! XD your coice.

4th you still want more health and enemy's arent that op yet so no ned for armour just get warmhog's armour its a must with cho dont think twice just get it.

5th now is time for defence... if the enemy is balanced get guardian angel if they arent, see "alternatives"

5th-6th depending on the enemy you want force of nature and thornmail (if ad hurts you more get tm 1st if ap does then get fn 1st) thornmail gives you great armor and hurts dps champions really bad (i remember playing nocturne and attacking garen while he was running away... i killed myself...he had thorn mail). Force of nature is beast! it gives you a hell of alot of health regen and the most magic resist there is.

skill sequence, get rupture 1st it's the best while playing cho tank... (read how to play)
vorpal spikes is quite usefull when farming... nothing more (unless youre mage)
feral sream is really useful when ganking
feast well its just awesome ;)

I take exaust and ghost beacause theyre the most useful when tanking exaust is used when rupture isint enough or when the enemy is killing your ally

Ghost is great for many reasons either to run away or go nom nom nom or even to land a rupture.

to have 1st blood make sure your both in the bushes then once the enemy aproaches target the shquishy one 1st use rupture when you can then you ally attacks him as he's airborne he will then be unable to flee. If he does not die use exaust so ur ally can finnish him off.
(ALWAYS USE RUPTURE! IF U MISS DONT ATTACK!)

When playing cho as a tank you ALWAYS must be with a good attacker.

Here is a short guide on how to kill:
Single enemy:
1- spot an enemy thets alone
2- ask someone to hide in the bushes
3- go up to the enemy alone
4- attack him head on ( you wont be doing any damage to him so he will try to kill you)
5- use rupture and feral scream (he will be unable to do anything)
6-ur ally comes out of the bushes and kills him (the enemy is silenced and slown he will panic and die) belive me it works ;)

ganking:
1- jump in
2- knock everyone airborne
3- while theyre airborne use feral scream
4- take damage
5- laugh as the enemy tries to kill you
6- dance
7- win
8-be awesome
9- omg ur invincible
10-ok... dont get too cocky -.-"

Pros:
-Great tank
-Cannot die if levitan at 20 (seriousely i chsed someone past10 turrets and 3 attacking me and only lost 3/4 of my life
-fast (unlike maokai)
-great abillities

Cons:
- dont even try to go 1v1 you cant deal dmg