Just Lasthit it. If an Enemy comes in range for your Q, shoot him. At level 6, spread mushrooms at the ward positions and the bushes. Also i recommend a "safety Kill" and an "escape" mushroom. At Level 3, after you make damage with Q and E and are more speedy with W, you could play more greedy.
Game is going to be much easier, if you own Malady.

The Mid Game is TTT (Total Teemo Time). Play greedy, make Lasthits, hunt down enemys and run where ever you are needed.
If you have "Bilgewater Cutlass", you can go into 1 on 1 fights, you will heal yourself up and won't take much damage (at least not enough).
Make sure you always place a mushroom at your enemy, if you are in a fight, it will heal you, make damage and safe your way with the slow, ifyou have to escape.

After TTT, Late game will be tough. You should prepare yourself invisible in a bush in the near of an coming teamfight. Always attack at teamfights as the last one of your team. Make sure they don't focus you. Normally, they won't notice you, after the first 3 guys are down.
Just be careful and you will win the game.