This is my teemo build. Ive went many matches goin 25+ more kills with less than 10 deaths and i've found this build works very well for me and hopefully for you too

When playing ap teemo remember you are very squishy, you can't take a whole lot of damage and until you get lvl 6 if you know you can't directly assault another champion, kiting is your best option. Learn to use your camouflage and if your mid, use it to harass the enemy champ and gradually weaken them. By the time you reach level 6 start placing shrooms around your lane to protect yourself from enemy ganks and to slowly weaken the enemy champs. It's good to always have your Fast move skill on, you can easily run up to a dps champ, use blinding dart, pop off a couple shots and retreat. Hopefully your poison will be doing enough damage that it will force the enemy to recall earlier so minion farms will be much easier.

Mid game is vital if you've boughten mejais. Team fights should start commencing and well placed shrooms should help slow and injure the other team. Placing mushrooms behind enemies to cut off retreats can get you assists and kills to raise your stacks. Always remember to stay hidden and use your camouflage if you know you are cut off, if your lucky you may be able to pick off an injured champ as they retreat towards their tower. Always initiate with a blinding dart on the Ad/dps champ and stay relatively behind the tank or higher hp champ

By now if you haven't died often and have been gaining stacks you should be able to 1v1 almost any champ. A special tactic before fighting is to place a shroom right in front of or behind who you are targeting to initiate a slow before the fight starts or as they run to ensure damage and slow.

*If you find yourself needing more AP/pen sell off malady for an item of your choice (Zhonyas hourglass, Abyssal Sceptor, or Rod of Ages)*

Lich Bane, Rylais Scepter, and void staff are also viable options

Pros/
High damage output for mushrooms
High AP + Attack speed for poison
High damage poison shot
Godlike Run speed with ghost + move fast
Decent CD reduct
Good late game
Very high mana regen

Cons/
Low Health
No negative spell block
Easily focused early on