Hello, i'm a 1600 Elo player on EU-West and I want to talk about AP Teemo.
Most people prefer to play Teemo as a bruiser or on-hit, but after trying out all builds I must recommend going AP.
The reason i don't like bruiser or on-hit Teemo, is because Teemo is inherently squishy.

Because Teemo is a squishy champion with only a minor speed boost skill(W), and excels in harassing his opponent, it's important to know if you're getting ganked while overextending.
I start by buying a flask, 3 health potions and a ward. Go to the lane and place your ward when the enemy Top-laner arrives (or whenever you feel like it).

Since your Q is an auto-attack reset, you have to time it for maximum damage.
Shoot one auto-attack followed up immediately by a Q and an extra auto-attack if you can.

AA -> Q -> AA

It takes some practice, and is key to winning the lane.
If you are playing against someone who does lots of melee damage (Ex. Xin Zhao) retreat with Move Quick after using your combo. Rinse and repeat to whittle down the enemy's HP.

Don't forget to last-hit minions in between poking.

I'll be updating this guide frequently, so please leave suggestions and questions below.
Thank you - Blueviking