My usual AP teemo build (top lane)
1) Boots, 3 health pots
2) berserker boots, ward
4) Nashor's tooth
7) Depends on situation
If they have magic res, I get magic pen.
If the other team is tankier overall, I grab a GA.
If I need magic res, I go with Wit's End.
-I don't go with abyssal scepter because of the AS bonus of Wit's End.
If I need armor, I get Zhonya's Hourglass.
This build tends to be more glass cannon; however, if you situate yourself correctly, you won't be on the bad end of ganks or even in team fights. Sometimes I'll exchange Malady for an early Wit's End depending on the situation. Early AS with AP and pen runes on teemo is top lane gold.
Your E juxtaposed with AS and AP runes will give you an extraordinary advantage early game, so max that ASAP.
Your Q is amazing against top lane melee champs, so in this situation, get that during your first level instead of E, but still max E first afterwards.
W is handy for leaving or chasing, but it's not necessary to level it past the initial stage early game.
Level your ult for obvious reasons.
Also, Teemo bot is useless imo. There are many better options for ADC, and Teemo just doesn't fit the bill. Teemo is best as top AP. For harassing against a melee champ, get your Q first. Get close enough to them that you can auto attack, and do so. Usually, they will come at you if they think they can score some damage on you (if not a quick kill). Pop your Q right before they reach you, and retreat back. They will only land one auto attack on you, and it will miss. Rinse and repeat, and, after level one, your auto attacks will hit with E as well and cost them even health more for their mistakes. Buy wards after your first base run to avoid being ganked, and save your flash for that very instance. Avoid flashing due to a failed combo, you will usually make it out alive.
After level 6 lane phase becomes a joke for you because you can just place a shroom in the middle of the minions and profit from not only an increase in CS, but also your opponent will most likely back off or take damage. Since you are ranged, this situation is cake.
To look into a ranged vs ranged situation, being aware of your opponents actual range is key. If you're against someone like Vlad, a champion who can out-range you with his Q, then potions, as well as an early E is necessary. DO NOT buy health items early game. You are only wasting your gold and setting yourself up for future failure. To poke, just auto attack every one in a while.