Just made a major update to the guide. Almost everything is changing.
As Teemo, I suggest top, or if that's not available, bot. Many people underestimate a Teemo top, because "he's squishy with no damage". Wrong. Teemo is okay health-wise, and late game, 2.5 attacks per second, with poison and slows every auto attack, Teemo has extremely high damage.

1st item- Malady's. Really good for the AS and AP.
2nd- Boots. Who doesn't need them?
3rd- Frozen Mallet. A slow every basic attack? 2.5 basic attacks per second? Each one dealing about 120 damage? OP.
4th- Wits End. Good for MR, and AS. The extra damage is nice too.
5th- Ionic Spark. Good for AS and splash damage. Some people may rush this; the damage can be a lot early game.
6th- Rabadon's Deathcap. Great AP, makes Teemo's poison much better.

Last-hitting as Teemo is easy. If you're a bit early, your poison will finish the minions off. However, I find Teemo a hard jungler, and don't suggest jungling him.

This build has been giving me good KD's for almost a year now; it should do the same for you.