Farming is very easy with cho because you don't only have your Q and W to clear hole waves, but you also have you E witch makes last hitting on lane very easy and you can poke with it. When your enemy isn't on your lane i recommend to push your lane to the enemy tower. If you have enough hp and mana to stay on lane you can make the enemy wraith, your wraith or gank. If not you should port back.

While laning you should always try to play safe. Poke the enemy with E's or W if you can lasthit a minion with it also. For engaing start with Q into W into as much E's as you can. When you have reached level 6 and your enemy is 3/4 to half HP land a Q(it will slow), go a few steps to him. Ignite and W maybe an autohit and the finish him with your ult. If he flashes before you can cast your ult dont be affraid of flashing after him. But be carefull to not get barrier/heal/double jump baited. With this tactic you should be able to get a few kills or a much better farm then your enemy early game to carry your team. And remember to always check your ult cd and stacks. If your ults isn't on cd and your haven't 6 stacks, feast a minion and then base, because your ult cd will be nearly ready when you come back to lane.