Ok , so this is my first build I made . I 'll try to explain as much as I can and I'll hope you players like my way of playing Teemo .

I'm not great in Introduction's so we'll end this fast and go to the build !

So , early game .
With the masteries and runes I set when I play Ap-Teemo , you'll have around 45 - 55 ap when you start the game . So that's already quite a lot .
With that much ability power you're blinding dart and toxic shot will hurt the enemies for sure .

One of the greatest things of Teemo is his stealth for sure .
When you know you can hurt the enemy badly you should stealth and wait for the enemy to try and get firstblood . Ofcourse you don't have to take risks while trying that .

Try to kill as many minions as you can in early game so you can get your shoes as fast as possible .

When I play AP Teemo , I always start with Doran's ring .
It gives you some abilty power , mana reg and health so you are a little bit less squishy .
The second item I go are sorcerer's shoes just at the fact it gives you magic penetration which is very good for Teemo .
3rd I go mejai's . Do this only when you are a bit of an experienced Teemo and know how to play it without dying much . Put your shrooms on good locations where enemies will walk in them for sure so you get a lot of assists to stack up your Mejai to gain great ability power . When you reach 20 stacks your ability power will have increased a lot and your cooldowns will be reduced with 15% . You will be able to put a shroom in approximate 16 seconds.
4th , a must have , Rabadon's . This will increase your ability power a lot ! Your shrooms will be doing massive damage and your blinding dart can hurt an enemy badly !
5th , Void's Staff . This will increase your ability power even more and the magic penetration makes sure your enemies will get hurt ! Because of this item even tanks will feel the Wrath of Teemo .
6th , Lich Bane . If the enemy-team didn't surrender by now because of you owning them , you go lich bane . A great item to increase ( yes , it still can increase ) your ability power . Lich's passive is great as well . You're next magic damage -attack will deal bonus damage to crush your enemies.
Finally you buy Deathfire Grasp , it is an awesome item IF you use the active . It hurts the enemy with 30% of his total health .

That's it for the items .

Teemo is in my view one of the best champions .
He has stealth , Map overview and can beat the **** out of your opponents .

If you go lane with Teemo make sure you put a shroom in the little bush in the river so you see your enemies come when they try to gank you .
Ofcourse , you are able to do that too . Teemo is a great ganker .
With his Move Quick you can approach your enemy fast and hit him with your Blinding Dart that will blind him and use some normal arrows then so your poison from toxic shot can finish it .
Move quick is great as well if you need to run away .

When you play mid Teemo you have to make sure your shrooms are placed very well so you can see when you will be ganked .

Teemo is a squishy champion with not that much health .
In a team fight it's recommended for you to be a little bit at the side of it or to be in stealth so the enemies can't stun , snare or silence you. You have to help your teammates ofcourse .
Use your blinding dart and toxic shot to crush the enemies that are running low on hp .
( if there is a Master Yi , Caitlynn or another champion which is good with normal attacks , focus them so there attack is reduced to almost 0. Do that with blinding dart ofcourse.. )

So , this is my first build and I hope you guys liked it and understand my way of playing Teemo .
If you do exactly what I say in my build it's almost impossible to suck with Teemo and your teammates will be happy to have you on their team .

Enjoy !