This guide focuses on the pure AD side of teemo. Teemo is a wonderfull AD carry even without items like Malady and Wit's End. In this guide we are maxing his AD damage and still maxing the damage he deals with his Toxic Shot.
Sorry for writing mistakes, my native language is not English, pls leave a comment when/if you spot mistakes. They are appreciated :)

Pros
+ Has a blind
+ high movementspeed with Move Quick
+ High damage with Toxic Shot
+ you are a yordle

Cons
- Squishy early game
- short range
- that's kinda it xd

I picked the Mark of Attack Damage runes because they are crusial to every ad carry build. They give you alot of poke early game and they will help you win your lane.
The Seal of Armor runes are for survivability. You will face an enemy ad carry in your lane (most of the time) so you need to be prepared.
The Glyph of Scaling Mana Regeneration provide a good amount of mana regeneration when you reach lvl 6 and need to start puting down some Noxious Trap
quint of attack damageI chose the quint of attack damage runes for the same reason as the mark of attackdamage runes.

The Long Sword is a beautifull item to start with. It gives you +10 AD damage and it only costs 400 gold so you can still buy 2 Health Potion. It will give you poke potential and survivability.
Rushing your Phage (after your Boots ofcourse) will give you AD damage + Health (that's very helpfull because now you can take a punch or two ^^) + it has a chance to slow your enemy so you can get the last hit on him/her when chasing. After you get your Berserker's Greaves you'll need your Vampiric Scepter/ Bilgewater Cutlass now you have some regeneration after you just owned the other carry with your Phage and the Berserker's Greaves will provide a nice amount of movement speed + attack speed. Attack speed is one of the most important stats for teemo, so we build a Statikk Shiv after your Vampiric Scepter for some extra attackspeed. The passive is also REALLY nice because you hit up to 4 targets at once! You can also build a Runaan's Hurricane. It let's you put your poison and the slow from you Phage on multiple tragets and it's also better to use when the enemy team is starting to stack Magic resistance because your Statikk Shiv deal extra magical damage.
This will give you a big advantage when fighting multiple enemies or in teamfights.
At this stage of the game (mid-late game) people are building some armour, so you want to counter that. After finishing Blade of the Ruined King (I prefer this item over Bloodthirster becasue it gives extra attackspeed and the active is better in fights than just the lifesteal from the Bloodthirster) you'll want your Black Cleaver. At this stage of the game (mid-late game) people are building some armour, so you want to counter that.
Now you can choose between Infinity Edge or Guardian Angel. I prefer Infinity Edge first because it gives you a ton of damage and a little bit of extra critical damage.
The Guardian Angel is so you can die and get away with it... literaly

Maxing you Toxic Shot first will give you a nice amount of damage to poke the enemy AD carry early game and it gives you a way to bypass the armor of the enemy tanks.
The second skill you want to max is Move Quick this will give you passive movementspeed. This will help you with chasing, escaping and even farming, in other words it amazing.

camouflageCamouflage: this is teemo's passive. This passive is amazing but hard to master completly. It can help you escape and set up ganks. The attackspeed boost gives you an advantage over your enemy.
Blinding dart: Blinding dart a blind that you just have to click on your enemy. !you can only blind 1 target! so you'll have to think before you use it, blinding the wrong target can ruine a whole fight.
Toxic shot: This is teemo's best ability! It's a passive ability that gives you extra (AP)damage on impact and it put's an overtime on your target every time you land a basic attack.
Noxious trap: this is teemo's ultimate ability. It doesn't look like much when you place this but when you place them on key points they will give you kills or safe your life.

-Teemo is an amazing ad carry, but he is not the easiest ad carry of the game. He requires some practice to master him as an ad carry, but when you do he is amazing! You can easely handle the enemy ad carry with your blind and even poke him/her.
-Teemo has relative short range so positioning is very important.
-Always try to make an escape route with your Noxious Trap when you see a teamfight breaking out.
-Try not to waste your blind on AP champions, use them on there carry or champions that use alot of basic attacks.
This was my guide for AD carry teemo, hope you learned something :)
ps. this is my first guide

Teemo's role in this guide is AD carry. This means you'll have to do the mose !AD! damage of your team. You'll need to put as much damage on your ememies as possible (constant damage is better than burst). It's not easy to play teemo ad carry because he has short range and no flashlike ability so positioning is very hard, but not impossible :).