I Personally play 21-6-3
You can greatly harras with this setup because you got some mana regen for more Noxious Traps, magic pen for your Toix Shot and the armo pen because you are going to be AD Teemo.

Early Game:
Only lasthit minions, get as much as you can but don't forget to harras the enemie champions and hold them away from their own minion kills.

Mid/Late Game:
Use your mushrooms to kill the caster minions, keep lasthitting the other ones.

Jungle:
When you change lanes kill the creeps on your way, it's some easy money.
Pick up the blue buff, so you can place your mushrooms more often.
You don't need the red buff so urgent because you got the frozen mallet for the slow, if no one else wants it take it for yourself to get the extra damage.

Teamfights:
Place your Mushrooms when you are able to, for the reason it's AOE it will deal great damage to the enemy team.
Dont forget to use your blinding dart, search for the strongest AD hero or simply use it on a ranged AD hero of the enemy team to disable him.

Stay in the back! A dead Teemo isn't a good teemo. When you get focused activate Ghost and Move Quick and run away, but come back when they stop to focus you and join the fight again.

"Shroom routing"
Escaping:
Place your mushrooms on some escape ways, if you lose a fight you, and your team, can flee on this route, you can't get chased good because of the slow.

Kiting:
Place some shrooms on a route. When chased by enemies you and your team kite them trough the mushrooms. They get damage and slow. Your team can now easily use their abilities while the enemy is slowed, they won't find a way out.

Look at your team:
So, if you want to do first blood, you MUST have 2 stunners or 1 tank and 1 stunner, NEVER go if you haven't got that in your team because teemo can't realy resist at the first blood with 450-500 hp and can't stun.
Look at the ennemies champions:
If they're 2 low life champions like mf, teemo, annie, etc... ok you can
If they're 2 tanky champions or caster champs like blitzcrank, volibear, singed, renek...don't go you're going to take an ace even if your teammates are jax or trynda or what op champions you want. don't go!
If you can go at first blood ( up conditions):
You have to focus one ennemy ( low life champions first) you must hit him and run away, always hit/run away , hit /run away, (trust me poison helps a lot, that's why you have to take the poison first) etc... don't try the 1v1 against a melee ad, only hit and run..
BE SMARTFULL! teemo is not realy a carry ad so that's why you've to play with your brain and not always want to attack like all teemo I saw, adapt your style to the situation.