Hello guys, this is my first guide for my favorite champion: Teemo.
Teemo is a champion that can be played in make roles, he can be played as AD carry, AP (which is not recommended),hybrid,DPS.
In this guide i build teemo like a BAWS :)
Combining his E skill: Toxic Shot with some items like Wit's End , Madred's Bloodrazor he can deal much dmg per second.
So let's start...

Your starting items should be Boots and 3 Health Potions
Or if you laning again something with high AD at early game you should buy Cloth Armor and 5 Health Potions .
When you go for 1st time back to shop you should have enough money to buy Vampiric Scepter and 1-2 Health Potion if needed.
When you hit level 6 you have the Ultimate Map Awarness, just spam your ultimate Noxious Trap whenever you have it "UP" and full you lane with shrooms.
Try to get red buff when you can,it helps so much.
Next time you go to shop you must complete your Wriggle's Lantern and possible your Mercury's Treads.
Camp in top lane and farm untill mid game arrives.

Here we are...
Now you rush for our best item Madred's Bloodrazor, this item combines with E: Toxic Shot and Red buff making teemo a Killing Machine.
Now it's time for some health so we build Frozen Mallet.
Getting Health,Damage,Armor,Slow making teemo unbeatable.
Don't forgot that you must run-hit-run-hit don't stand still.
Slow from Frozen Mallet and Red buff make you able to escape gangs and chase enemys.
In this Phase you have already left top lane and you helping other lanes as/when you can putting shrooms arround the map etc.
Try to put shroom arround baron,enemy buffs,dragon,in river,in bushes everywhere

Now it's your time to decide what you build next: Wit's End or Guardian Angel
If you see that your survivability is low, build Guardian Angel other way build Wit's End.
I don't have to say many about Late game just care!
Teemo is realy weak in late game, all you have to do is: Push lanes like a baws and put shrooms (as you already do).

From masteries we are running 23/6/1.
Fron Offence:
Getting 3 points on Brute Force to make last hits more vailable.
4 points on Alacrity and 4 points on Sorcery,need cooldown reduction for ultimate.
1 point on Weapon Expertise and 1 on Arcane Knowledge , need Magic penetration for poison and shrooms damage.
2 points on Havoc and 3 on Vampirism
3 on Sunder and 1 on Executioner
Fron Defence:
3 Points on Resistance and 3 on Hardiness
And from Utility:
1 on Summoner's Insight for cooldown reduction on summoner spell: Flash

From Summoner Spells we are using: Ignite and Flash
Also teemo is vailable with Heal (not kidding) , Exhaust and Cleanse.
But never pick Clarity, Surge, Revive they're realy useless

As i said this is my first guide, hope i help you guy.
Go Go Go play Temmo!! Have Fun and "Never Underestimate the Power of the Scout's Code..." :P