This guide is meant to be informational on item's to buy for Teemo and skill sequence.

The straight AP runes will help you harass with Q early and get a couple ganks early, which is when you need to get fed. It will also help a little bit late game.
These runes pay for themselves by feeding you early game. Getting fed early is the most important thing you can do when playing this guide. Early Feed = More Money = More Items = More Kills = More Turret Kills = More WINS.

These masteries are straight up AP and extra damage for extra help early game. That is when you need to get fed.

Sell "Doran's Ring" after you buy "Grez's Spectral Lantern".

My items are a mix of AP and attack speed, because Teemo's Q mixed with basic attacks and E will deal major damage quickly.

Maxing Q with your early AP will help in the early gank/harass game.
Take a stack of W early just in case you need to run.. FAST!
Be sure to use Teemo's R in the jungle.. A LOT. It will help tons when you need to run from or chase enemies. You can also use Teemo's 'shroom as a ward of sorts.

Ghost mixed with Teemo's W will make you FAST. I pick this over ignite because with ghost, you can catch up to the runner and get another basic attack or two, dealing more damage than an ignite would.
Flash is simply for use against any champion. e.g. jumping over Rammus's "Powerball"; juking; dodging Ashe's arrow, etc. Oooorrrr for you not so cool people, you could just hop a wall or something. ;-)

Be sure to last hit and try alternating hits between minions. You'll notice your poison helping you to "last hit" the minions for extra gold.