For Marks i take : Physical damage
For Seals i take : Cooldown reduction
For Glyphs i take : Mana and Cooldown reduction

I take masteries in ''Offense'' because, so i also can do some normal damage.

In the start : I buy shoes, 2 HP potions and 1 Mana potion.
Then i start buy : Berserker's greaves ( For some attack speed )

After Berserker's Greaves i buy Nashor's tooth ( For some attack speed and AP )

After that, i start buying Hextech Gunblade, it is a very good item for Teemo!
It gives you attack damage (60)
Ability power (75)
Life steal (20%)
Spell vamp (25%)

Then i buy Rabadon's Deathcap, because of the high AP
It gives you 155 AP.

Then you can buy Rylai's crystal scepter (If you wanna beat out some guys, with higher HP)
It gives you : Health (500)
Ability power (80)

Otherwise if you wanna hit more, then go to buy a Rabadon's deathcap og a Hextech gunblade again.

Hey..

If you liked my build, then feel free to comment and rate my Build.

Thanks! :)