Cho'gath is a pretty good mage, but he suits the role of tank better imo. I've played cho'gath a very long time and it hurts my heart to see random 10 year olds build him wrong. This guide will cover the basics about building and playing cho'gath ( this my first guide, be nice )

I go with armor reds, dodge yellows, magic resist blues and two dodge quints and 1 armor. this set up helps me lane without getting to harassed and it's very efficent late game as well

I go with the build i've always been using on cho, start of with boots 3pots, and quickly get a glacial shroud (sometimes if you are facing someone that have very good ranged harass like yorick, u should consider getting a gold per 10 item), this can differ if your laneing vs a mage it can be better to get catalyst before shroud and maybe even complete mercery threads. The rest is pretty basic,roa, force of nature, guardian angel and the last item is really up to you with team your meating, sometimes i go guardion angel to avoid heavy burst spells, sometimes i get wits end to make me hit a little harder and get the extra magic resist needed (abyssal scepter also very good item for this) but most of the times i get quicksilver sash so i can get out of cc fast and help my team with a silence or rupture. But as i said, it's very diffrent depending on which team your facing. ( if no1 gets a aegis of the legion it's also a decent item)

I often start with feral scream since it does pretty good damage and silences every enemy that it hits, it's very good when your counter jungling with your team since it can stop some1 from flashing away, or nuking down the support etc etc, i try to get rupture in level 2 since i often get early ganks top and it is really usefull when trying to catch up with someone running. After that i put one point in vorpal spikes to help me last hit and do some tiny harass if the enemy is close(remember to toggle of when attacking tower close to a champion). After that i max feral scream since it gets longer silence the higher rank it gets, and then i level vorpal spikes to level 3 or 4, depending on how many last hits i have, and then max rupture, and last vorpal spikes (ofc pick feast at 6,11 and 16), feast minions as often as you can, tho' if u think a teamfight is coming up it's good to save it.

Early game, try to last hit as much as you can (ofcourse) , when you get ultimate, try to stack it to max as fast as possible since u will sustain much better. try to have map awarness if a team fight is about to start you will do much more use silencing their team then standing and last hitting the few waves that will appear before ur opponent gets back. If you have someone like malzahar or annie you can tp to their minion but otherwise teleport to closest creep or tower and run there. Also in lane, when you can get red, it helps you very much with harassing and assisting jungler during ganks. Tho in mid/late game, it might be better to give it to the ad carry. Late game when you have your resistances and fully stacked rank3 feast, u can pretty much stand and soak every attack from their team so always ALWAYS run first and engage, try to hit as many people as possible with your silence, save rupture untill someone is trying to run away, (or engage with it if their out of range for silence) feast a weak champion so your team easy can finish him/her of ( preferably the ad/ap carry, in worst case the support ) cho'gath is a very strong champion so make sure you take use of everything he can do.

Cho'gath is pretty weak early game in compare go other top lanes,but when he start stacking feast he is one of the better. I've also played him mid a few times, it's not the best but it works pretty good, that's it for me, be nice, adios :)