Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Teemo carrying his team in his backpack
Teemo's backpack consists of boy scout utensils, graham crackers, cheese, juice, and 4 other champions on his team.
-Farm minions, get your items fast.
-Place your mushrooms at key locations on the map. Early game put them at bushes to help your team mates run from ganks or such, and place some shrooms at the buffs.
-You can also use your shrooms as a teleporting target, I use it effectively to run behind pushers and gank them.
-Teemo is a very squishy character, so play him safely and he won't die much.
As far as getting kills goes play teemo agressively only if you know you can get them, and are aware they won't get much backup.
During team fights hit the melee champs with your blinding dart to stop them from hitting you first. Then just get in range to spam your basic at people. If your team is there to support you then the people will drop fast.