If you are not new to league don't worry about this section.

Listen here summoner. The most important thing in lane is not getting those kills those will come in time. FOCUS ON YOUR FARM. Last hit as many minions as possible while paying attention to your enemy and not getting ganked. If there is an opportunity to kill your opponent then do so, but do not go balls deep because you will die.

You can go:
Boots + 3hp
Cloth Armor + 5hp
Or Dorans blade

By mid game you want to have
Phage, Lantern, and Boots.

Late game your goal is
Frozen Mallet, Wits end, and GA.

Your 6th item is up to you, if the enemy is stacking armor get last whisper. If they are heavy AP and you are being bursted get either banshees veil or Force of Nature or Maw of Malmortius. If they have heavy AD damage you could build Atma's impaler or Frozen heart. Or if you just want plane damage build Bloodthirster, black cleaver, or Infinity edge.

If you want to play teemo do not play him bot, he is outclassed by other AD carries and will not do well. Teemo is good against melee champions because they have to get close. With your slows + blind and W, you can easily kite top champions and dominate. Also you have your ult for those sticky situations. Use your shrooms to ward bushes and on the sides of the lane so that you cannot be ganked. Remember to focus on your farm and try to either play really aggressive for first couple of kills and dominate your lane or passively and wait for your jungler to gank.

If you are doing well start ganking mid and help pushing towers once top tower is down. If you are not do well continue focusing on your farm on top without getting killed.

A lot of Teemo players downfall is they focus on warding up the jungle and get killed. Stay with your team and help get draons+baron. Yes ward but do not do it if your team is pushing or trying to get dragon or baron. Also if your AD carry has red buff and there is one available it is nearly impossible to get away from a teemo with mallet+w+redbuff.