the bad thing about cho'gath is that he can't farm easy at lvl 1 by the low atack speed and enemy champion, but using his rupture can make more easy to farm and poke enemy but be careful for the jungler, don't advance too much with no vision on the river. A good combo is with Q and W to make the enemy more easy to kill, another combo is with Q,W and R if the enemy is low(you can use Q+FLASH+R to get a kill). be careful with your mana if a jungler gank you will need it to run from him.when you get to lvl 6 use your R to kill minions to get the stacks or (if the lane is easy) use just to kill the enemy. when ataking turrets disable E if a enemy champion is close because it will make the tower atack you, if it has no enemy you can enable it to farm at the same time

I use teleport because cho'gath don't have a good move speeed so it's help to get faster on the top lane or another place. use teleport when you get enough gold to buy the catalyst or the mana gem.

help the team poking the enemy or silence an assasin like kha'zix or master iY or an mage like leblank to make impossible to her to get back from a poke, same too lee sin

to farm better you need to get at lvl 2 to get vorpal spikes to deal extra damage on the minions, using rupture just if the enemy is too strong for you, try to poke and farm with Q at the same time.