Why These Choices?

Spells
Ignite to increase the chance of a death after a fight and flash to get out some sticky situations.

Items
Boots and 3 pots as a standard start, i rush the attack speed boots to take advantage of the bonus damage from Teemo's e and the damage form the masteries, The blade of the ruined king is there for a bit of bonus ad and life steal but also adds more hit on effect damage for the quick single target take down. The bonus cd reduction enables you to poke at late game with your q and basic attack combo to drain the of hp before the fight. Whits end gives you a little more survive ability going into the late game but also more hit on effect damage. Finally the rabadon's deathcap for the AP boost in damage on your q, e and r.

Abilities
Maxing e is good to keep the damage going out with the bonus's from the items. i think taking a point in q earlier is helpful at top as most champs use close range basic attacks which will reduce their damage to you. Taking your ultimate when you can and mushroom brushes to help keep vision on the enemies jungler if he/she wants to try gank you with out the cost of buying a ward.

Masteries and Runes
A little bit of survive ability and then very offensive from the 1st tree helps Teemo shine in the single target take down category as well as the AP from the runes.

Laning
When laning you have to know what their champ can do. Teemo counters champs that have to get up close and do there damage. this is the case in the early game and also in the late game if fed.