Cho'gath is one of the most difficult to kill tanks there is due to his ability to scale infinitely into the late game as well as his ability to practically one shot the enemy carry. He is great in team fights due to his crowd control, his ability to protect the carry, and his ability to basically erase one of the enemies.

Pros:
- Infinite scaling (resulting in a very powerful mid/late game)
- Lots of cc (knock up, silence, slow)
- Very hard to kill (can easily end up with 7k health or more)
- Does a lot of damage
- Can zone people out and force them to focus him
- Ult cool-down less than a minute at max rank
- Can use his R to get objectives
- You're a huge monster that can take up the whole lane and tower over everyone else

Cons:
- Very, very slow. This is his biggest weakness
- High cooldowns on Q and W
- R is often hard to use (this goes back to his speed and ability to chase)
- If you get too behind then you will have a very hard time coming back

Stacks:

I see people make this mistake all the time: not getting the minion stacks. I often look at a Cho and see that he only has 1 or 2 stacks at level 9, and none of them are from minions. that's unacceptable. Cho's power comes from his stacks and if you neglect the 6 FREE minion stacks you will be useless. The moment you get level 6 you need to get a stack off of a minion. In fact, in laning phase, you should be using your R on on minions any chance that you get (for the first 6 minions of course). Of course, you shouldn't use it if you know that a trade is coming, or if your jungler is about to gank. But at any other moment you need to use your R for those easy stacks, because once you hit 6 stacks you will hit a huge power spike.

P.s: it's smart to use you R on a slightly damaged cannon minion because you will guarantee that gold. In addition, if you're about to back, see if you can get that 1 extra minion stack before you go, because you will almost have your R back again by the time you come back to lane.

Combat:

In a 1v1 uour basic combos are:
Q -> W -> E -> AA x3, repeat until you can R
E -> AA -> W -> AA -> Q -> AA, repeat until you can R

Engaging:

To engage, use your Q to knock as many of their team up as possible then walk in with a silence and your E. this will get their team down low and prevent them from doing anything for a bit. If you have Righteous Glory then run into them with it and try to get to their carry.

In a team fight, target their carry. Use your combo on him and try and R him. Even if you don't kill him, you will get him down low and force the enemy team to target you instead.
If your carry is being targeted, protect them with your Q and W.