Im gonna be short and sweet here, these Cho'Gath builds are how I play cho'gath and is subject to change, the stats on the build aren't accurate either since Mobafire includes all the stats of every item on the list and does not consider the transition (includes the mresist of Negatron and FoN but should only have the mresist of FoN). Both Cho'Gath builds ensure that you have one of the best lane sustainabilities in the game while being almost impossible to gank. The first cho gath build focuses more on you being the big presence in team fights that can not be ignore and in a way has an anti-carry type of play. The second build focuses more on the offtank part of chogath while still retaining lane sustainability and cc.

Start of with a faerie charm, 4 health potions, a ward and 2 mana pots. Rush first Philo and buy boots of speed. Second trip back should be another philo. Also get the HoG early to get the gp5 then build it to randuins later. Catalyst to RoA is the next priority and get Mercs treads. By this time no one really has the ability to get you off lane unless its 4+ people since you have regen+teleport. FoN>Randuins in terms of tanking earlier on since most AD carries aren't done with their builds since they don't have the gp5 earlier on. Only sell the gp5 items until you know you are safe enough to keep farming for the last few items, warmog/guardian.

Same starting items as build 1 except this time you are not getting the HoG for randuins. This build focuses more on the slow effects of rylais and chogath's general nice base damage stats on his skills. Boots have been changed to sorcs to give that extra umph! to your damage. Merc's effects changed to Eleisa's miracle for easier sustainibility since no warmogs or randuins. Guardian angel is a bigger priority now because it's the only armor item you'll get. FoN can go at any time in the build.

Your purpose is to farm enough early and mid game while giving that constant lane pressure to the opposite team to prevent focusing on other lanes. Give that extra pushing power to balance out the fact that early-mid game you're not helping on ganks or teamfights. FARM,FARM,FARM!You should join in teamfights as soon as you get FoN and Randuins or FoN and Rylais. Get anough stacks and stand in front of your carries. There's a high possibility that you filling up alot of the screen that the ad carries would actually misclick you. Cho'Gath's taunt is in his size.
Don't be afraid to take turret hits but don't bait your team to stand near enemy towers either.