This build will allow Teemo to do some heavy damage and live long enough in team fights to lock down the enemy carry and (hopefully) get a few kills along the way :P

Pros

Blind
Schrooms
High Mobility
Great Map Control/Awareness

Cons

Shorter Range Than Most Other Ranged Auto-Attacker's

Greater Mark of Desolation for more damage early game
for more damage late game due do Poison Damage + Bloodrazor + Malady
for a harder to kill Teemo
Greater Seal of Vitality can also be used if you don't have the dodge ones
for a solid reduction of most Harrass Damage early game
will help you Escape/ Chase and run to your friends for a Team Fight/ Gank

I like to take as my first item because it allows you to stay in your lane for a very long time. On my first trip back (Hopefully 715 G) grab a Philosopher's Stone >>> and if you can a Sight Ward This will allow you to have a greater G per 5/ sec generation over probably everyone in your game at this point and also fixes most of your mana problems. Then, try to get more mobility from the early, for overall benefits with Survival/ Harassing Ability. On the next trip back (1000 G) for an increase in your farming Abilities/ Lane Pushing. Next up >>> ? <<< its up to you.. If you want more survivability and a chance to slow your target or if you want to attack 40% faster for increased damage done through and an increased chance to proc . It is your choice ( Sight Ward ) >>> (could probably be very helpful to your team at this time)... but no matter which you pick first, make sure to build the one you didn't on your next trip back to base.

Teemo has a nice harass combo when you use your basic attack once or twice and then throw in a You will also want to put your schrooms at the entrances to your jungle and in the opening to the Dragon and Baron to let your team know if the enemy is coming in your jungle or trying to take either of those two key neutral monsters. You can also place your in places that you plan to use during an escape (this tactic can save you and your allies countless times :P)

When it comes down to the time for some epic team fights, Teemo has one main job..to shut down the enemy who deals the most AD auto attack damage such as ect.. Make sure you save for them. If a team fight breaks out and your intended target for is nowhere to be found I recommend you continue to rain down pain but to still save for when your target shows up :P (its better to have your blind ready than to have it on CD and let your friends get picked off when decides to come in after most of the CC has been used up)

I know my build is short but it is that way because I don't want to bore you with a guide that takes 20-30min to look through..I just want to mainly give you my build and some tips to make your games with Teemo more fun and successful.
This is my first guide so plz let me know what you think and where I can make some improvements