So.... WHATS WITH ALL THE TEEMOTION?! This teemo build that i created is based on additional damage stacking, early game zoning, and late game durability and to put it simply..... PWNAGE!!! Your Toxic Shots, Ignite, and Malady will keep your opponent at bay and zone them early game; While your Wits End, Guinsoo's Rageblade, and Berserker Greaves will keep your opponents cowering from your attack speed mid game; And your Durability, and Life threatening items will have them write songs and create legends and tales about your late game skills. In the enemies eyes this is what happens durring the first minute of the game.
ENEMY:"I hope i get to face teemo hes so squishy" *the enemy walks around the mid lane looking for you*
Teemo(you): "CAPTAIN TEEMO ON DUTY" *you unstealth*
Enemy: "Oh god what the heck i though teemo was supposed to be squishy" *Tries to basic attack teemo*
Teemo(you): "Noob" *Blinding Dart*
Enemy: "What the?! I missed" *tries to use an ability*
Teemo(you): "You think that will stop me" *You use Heal*
Enemy: "NOOOOO! I gotta get away" *They start Running*
Teemo(you): "Time to finish of this Noob" *ignite*
Announcer: "FIRST BLOOD"
Announcer to Teemo(you): "You have slain an enemy"
Teemo(you): "Another first blood drawn by me because of this awesome build"
Announcer to Enemy: "You have been slain"
Enemy: "Nooo i thought i would kill him teemos are supposed to be squishy" "CURSE THAT TEEMO BUILD"

I take 3 points in Brute force for the attack damage at lvl 1 because it is useful for drawing first blood and important for your early game. I also take 1 point in Summoners Wrath because in this teemo build you will need ignite. I then take 4 points in Alacrity because in this build your attack speed will be one of the key components to your build. Of course i take the armor penetration afterwards for obvious reasons... Afterwards you should take 4 points in Deadliness for your early game crit chance possibility which then leads to the critical damage which will always help when you do happen to crit. You should always take the 3 points in Vampirism because it gives you lifesteal which is very helpfull early game if you need to stay in lane, this is helpfull for almost all AD based champions. I take 3 points in Sunder for the armor pen which is helpfull for penetrating their defenses late game. Lastly I take 1 point in Executioner because its just plain Destructive to the enemy.
In the defensive tree i take 3 points in Resistance because as teemo i always go mid because of my long range skills and auto-attacks, now if your going mid you usually face other long range champions and 70% of the time a mage. The magic resist is helpfull for staying in lane and being durrable against the mages. I take 1 point in summoners resolve because I use heal. I then take 4 points in Durability and 1 point in veterans scar because as teemo you are very squishy and need the extra health for durability.

I take Blinding dart at lvl 1 because the blind is usefull if your facing another attack damage character such as tristana, caitlyn, sivir, twisted fate, ect. I also like the damage output of blinding dart and the harass game(you should always go up and poke your opponent with blinding dart at max range then walk off). I then take Toxic shots at lvl 2 and 3 because the Poison is much better for your harass with blinding dart. I then take Move Quick at lvl 4 because if your in a bind you need an escape mechanism which it is very usefull for. Afterwards I max out Toxic shots as soon as possible while still taking my Ulti (poison Mushrooms) when i can. I then try to keep a safe balance between leveling my blinding dart and Movequick but take blinding dart first then fluxuate between lvling them. I still always take my ulti when i can.

Blinding Dart: Usefull for the Blind and is good for harrasing

Move Quick: a great escape mechanism and great for chasing

Toxic shots: Amazing for this build and essential because of the additional damage and damage over time which stacks with items in this build.

Poison Mushrooms: Great for laying down as a sort of sight ward and dealing masive damage to your opponents when hit.

I take Ignite because its simply a great summoner spell and can stack with your poison and get you first blood. And i take Heal because its good early game and can get you out of some sticky situations.

I use 9 Greater marks of insight because the magic penetration is very usefull for your poison which you can use to slowly burst your opponent. It is also usefull for blinding dart and the damage it does. I also love taking magic pen for my Poison mushrooms because of the immediate burst they do and the other Damage over time.
I use 9 Greater seals of Alacrity because the attack speed is a very big key in playing teemo because the more attack speed you have the more affective your poison will be.
I use 9 Greater glyph of Fortitude because teemo is a squishy champion and you need to build up his durability.
I also use 3 Greater quintesences of Fortitude for the same reason... Durability.

I take a Dorans blade as my starting item for the attack damage health and lifesteal which can prove to be very usefull if you need to stay in lane.
I then take a dagger for the attack speed which is just amazing when you combine it with your Toxic shots. I then buy another dager for the attack speed which will later be used for your Malady.
I lead my first dagger into my Berserker Greaves because the movement speed and attack speed are very helpfull in this build.
I lead my second dagger into a Malady which should cost you 1405 because of your earlier dagger purchase which leads up to it. Your malady will prove essential early game because it stacks with your poison and gives your attack damage an additional 20 magic damage and depending on the level of your toxic shots a lot more damage. The ability power from malady is also helpfull for our blinding dart, your toxic shots, and your poison mushrooms. It also gives you attack speed which again can help your toxic shots because the more attack speed you have the more usefull your toxic shots are.
After malady i get a Recurve bow because the 40% attack speed is always helpfull.
I lead my recurve bow into a Wits End which should then cost 950 because of your previously purchased recurve bow. Your Wits End gives you 40% attack speed which again gives your toxic shots a lot more usefullness which i cant stress enough. It also gives you 30 magic resist, and each basic attack grants you an additional 5 magic resist and can stack up to 4 times which you will need to battle those pesky mages. It also gives your basic attacks an additional 42 magic damage which will stack with your toxic shots and your malady to give you massive additional damage.
Afterwards i purchase a Blasting wand for 860 because it gives me 40 ability power which helps my abilities and especially toxic shots.
I lead my Blasting wand into a Guinsoos Rageblade which gives me Attack damage which is all the better for your crits, ability power which is again good for your abilities, and has another on hit effect which increases your attack speed and ability power each hit.
I purchase a Phage next for 1315 because later in the game you will need more durability and your phage will give you that with health and an additional 18 attack damage.
I lead my Phage into a Frozen Mallet for 1935 after phage for the slow and durability.
Finally i purchase a recurve bow for the attack speed which i then lead into a Madreds Bloodrazor for The 4% of the enemys health each hit which is just lifethreatining and will finish off your build with a bang.

Teemo is a great farmer wants you reach lvl 6.
When there is a creep wave you can just set your poison mushroom down in the middle and watch the poison do the work for you.
Teemo's attack speed will prove amazing for last hitting minions.