There are two meta Teemo builds: Fighter Teemo, and AP Teemo.
Max: E, Q, W
Start with: Boots, HP, then grab a Phage (for early game sustain, slow, and damage), you can head for boots here (Merc Treads / Ninja Tabi / Zerk Greaves) or proceed right to Wit's End. Then get your Frozen Mallet, and you'll be all set for team fights (additional items are your choice; for more damage get Madreds Bloodrazor, for more sustain get Hextech Gunblade, etc)
AP Build, aka "The Mushroom God"
Max: Q, E, W
Grab boots and hp (as always) and proceed to Catalyst, get Sorc Shoes, get RoA, then either Void Staff or Rabadons (depending on how much gold you have). You will be heavily criticized for this build, but it is actually EXTREMELY effective (if you have good ult placement).