Hello guys this is my first build so please go easy on the comment. I have been playing with Teemo several games lately and it is an awesome champions. Teemo can turn the games around if played right. I do use a few AP which is for Toxic Dart only.

DPS = Damage Per Second
CDR = Cooldown Reduction
AS = Attack Speed
MS = Movement Speed
AP = Ability Power

Greater Quintessence of Fortitude - It is quite useful in the beginning for some HP boost.
Greater Seal of Alacrity - I don't think I need to explain this :)
Greater Glyph of Focus - Teemo needs CDR so that he can use Blind Dart as fast as he can.
Greater Mark of Desolation - Since this is a DPS build, he is going to need for armor penetration.

I go for 21/9/0 for killing and survivability.
You can choose this mastery that I go for or choose your own.

Beserker's Greave - Good for Teemo, AS and MS.
Malady - More AS and some AP.
Nashor's Tooth - And more AS and CDR.
Sword of the Occult - More DPS as you get more kills or assist and at max (20) stacks, you get
extra 15% MS.
The BloodThirster - Great DPS and LS for survivability.
Madred's Bloodrazor - To me, this is the best item in LoL, because of it's Unique Passive :
deals mangic damage equal to 4% of the target's HP

Exhaust and Ignite ftw... thats all I have to say :P

I usually go top or bottom lane with a champion ally that can stun/slow/haste/snare/taunt such as Sion, Olaf, Singed, Ryze or Rammus, these are just some examples of Champions I usually lane with.

Go for Lizard's buff (red) and Mana Golem buff (blue) whenever possible so you would deal extra damage, CDR and higher MP regeneration. And your mushroom, always put it near Baron and Dragon (for ganking) and mostly bushes and river for escaping and killing.