Teemo is a fun champion to play in lol. Mostly becouse of his "noxious trap" and "Poison". i would recommend you to play the AP Teemo.

I chose mana regeneration Seals becouse you sometimes i ran out of mana.
AP Glyph and Quintessence for more poison damage at the beginning.
And magic penetration marks for more magic damage also.

At masteries i go 9/0/21
Most important is to the Archaic Knowledge for 15% magic penetration. Then also Intelligence for cooldown reduction and then follow up with the Presence of master for summoner spells. I upgrade Flash and Exhaust and Quickness. Those are the most important masteries for AP Teemo

At the beginning i buy Doran's ring for more AP and Health. Then I quickly follow up with sorcerers shoes for more magic penetration. Then the most important item is Rylai's Crystal Scepter, it gives you HP for more endurance, AP and slow which is a great combination for his Shrooms. Then i buy Rebandon's and Void staff for more AP and magic penetration. And then buy Banshee's Vail and Frozen Heart for endurance, mana and cooldown reduction.

I have to go with Exhaust to slow my enemy's at the beginning and Flash for a quick escape.