A first this build is a mid-lane build so your very good in mid.

1.) Buy Dorans Shield very fast and do teleport to the tower 4 in mid2. Run shortly out of the range of the enemy tower 4 and dont go away ( so that your passiv ability can work).

3. When your enemy comes wait when he is in near of your tower than attack him. He will become so much damage that you can kill him with luck.

4. When you kill him and have no much life go to your tower and wait. Dorans Schield will regenerate your life fastly.

5. If he kills you, you should think about whether you should really play with teemo :D

6. At teamfights you have to attack the damager with your blinding dart and kill them. After that you can attack the AP-Champs.

7. Lategame: Every time you have to be the last, who attacks because they will focus you if the can do it.

Why to take Dorans Schield:
You get a little bit armor, so you withsand more attacks at the beginning and you have lifereg. and more life.

Berserkers Greaves:
You are a little bit faster and you have more attack speed.

Nashor's Tooth:
YOu can shot your Darts faster and your "e" makes more damage ( all you abilities). And you have manareg so you havent go back when you are out of manacand the passiv makes that your abilities are ready faster.

Malady:
Your attack speed becomes faster and you become more ability power. The passiv makes that every shot do extra 20 magical damage and reduces the magic penetration for a short time.

Guinsoo's Rageblade:
You make more damage and have more ability power. Also the passiv is good in fight you become fast a little bit stronger.

Hextech Gunblade:
You get much ability power and a little bit damage. The little bit life steal is good in fights where you are foucused. So your skill gives you life too and you can do 300 damage on a champ with 700 range.

Frozen Mallet:
You have a little bit damage too and much life. Also you can slow your enemy.