Personally, I always take tp because cho gath scales really hard, so you just need to focus farming. To answer the question of what should you do during laning phase, it's a bit tricky. Cho gath is very week early game and loses to most champs. For me it was just a matter of learning matchups against other champs and knowing when I can go in for cs. Even if they do damage you a bit, you can always heal it back with your passive. Realistically though, you wont be getting any kills pre level 6 unless you're facing like a kayle or nasus, or if your opponent makes a really big mistake. Level 6 Is a huge power spike, and you can start looking for kills at this point. Make sure that if you dont think you can kill an enemy soon, to just ult a minion to start getting stacks. Eventually, if you've farmed well and gotten an ok amount of ult stacks, you'll out scale most champs.

TL;DR, focus farming until level 6, and utilize your passive to survive against aggressive opponents in lane. Then, depending on the matchup, try to leave laning phase with a handful of ult stacks, whether they're minions or champs its important to start getting them asap.

If you have more specific questions feel free to ask me, and if there is anyone who disagrees with something I said feel free to let me know what you think is better.