At the beginning you always start with doran 's ring and two potions. Then at your first back i recomend you to buy the stinger and the boots of swiftness because these gives you the indispensible cdr,attack and movement speed.

The fisrt item you need to buy is the nashor 's tooth. This is the most necessary item on
teemo because it gives you more ap power per hit ,attack speed ,cdr and ap.
After nashor 's tooth and the infinity attack speed you should buy gunblade.With this item you can burst the enemy adc and this is a good life steal source.

Rageblade is the third item.I add it on this build for the perfect hybrid power, attack speed and the ability power per hit.