first thing to know is that cho is a tank/ap/support hybrid and very fun to play with.

2ndly, it is best to feed cho... rather then feed cho to enemies....
cho'gath's ULT. is feast, which is best used on minoins and low level monsters.
try your hardest to farm "feast". this will allow you to focus on your build and not worrie about health items.
Now, movement is a problem with chogath, so i prefer "FORCE of NATURE" which gives
MR(magic resiest) and great movement.
also. EARLY game is always difficult for cho because of mana and cooldowns. to solve this i suggest NOT solo laning and help get kills with "RUPTURE" /note:
(the more champions/monsters you hit with rupture the more splash damage it does.)
AND buying the "FROZEN HEART" item for both armor, and 20% cooldowns. trust me.. it really helps.
now once u hit around lvl 11, you should have full feast stacks, and should be atleast half way through your build. follow your team around and nuke with RUPTURE and FERAL SCREAM. becuause they diliver mass magic damage,slows, and silence.
do NOT try to SOLO multpiple enemy champions....just becuz your big.. just makes u a bigger target. and nothing sucks more then having to rebuld your feast stacks.
being BIG will make you one of the PRIME TARGETS. so use rupture to get away if your being charged at. unless your teams behind you.. in which case... nke em, and attack from the mid
doing this will make you less likely to take as much damage. (JUST BECUZ YOUR HUGE, DOESNT MAKE YOU THE TEAM!- so team fight along with a buddy)

1.YUMM-
champion at mid HEALTH, use RUPTURE >FERAL SCREAM > FEAST.(recommended enemy health 300-1000)

2.NUKE-
at mid range (with team) follow in behind team and from the shadows use RUPTURE >*exhaust* > FERAL SCREAM > FEAST > RUPTURE >(repeat). (should take down atleast 2000 health)

3.WTF-
this is a harder combo to pull off, but at full stacks,HP, and full build.
use combo 1. then flash away, then continue to combo 2. (this should allow you atleast 2-3 kills depend on how OP and how many enemys are.

focus on AP, MR, MOVEMENT, HEALTH, and COOLDOWNS. best runes for Cho'Gath.