Teemo is a naturally quick champion who can build both AP and AD and excel

I've chosen to go with AD in this build as it works well with your passive.
This build has a bit of a slow start but will build into a map control AD carry with massive mobility.

I prefer to lane with a partner who can cope with you running off occasionally to plant mushrooms which you should be doing from the moment you hit level 6 (if you are carrying 3 mushrooms for more than a few seconds then your doing it wrong) even in a fight you should be placing mushrooms to prepare for falling back.

Your main role is to protect your team by placing mushrooms everywhere this will enable them to gank people walking around and also prevent them being ganked. In this build you will gain more gold than a enemy doing equally well due to the early Avarice Blade.
Getting kills early game without dieing will mainly depend on your partner and you wont be effective in lane until you get the Vampiric Scepter so take it easy early game farming and keeping alive. Remember you are a carry and you will need a team that can carry you at the start.

Pros
- good map control will make it so your team gets side swiped less and so less team deaths
- near the end you become a real heavy hitter dropping turrets/people really quickly
- with over 500 movement as standard once you finish trinity force you are hard to catch
- with your dart you can negate serious damage
- less need for your team to buy wards
- mushrooms are awesome

Cons
- very squishy
- the slow start can be a issue with some teams you are a carry so make sure your team can carry you at the start

As mentioned in the intro this build has a bit of a slow start and you will often get abuse from people who think first blood = game win as you are not really set up for first blood.

Take Brawler's Gloves and 2 health pots and goto lane with your parter (i prefer to avoid mid as this will limit your shrooming ability).

once you hit 700 gold i normally return for Avarice Blade and boots of speed this will help you keep alive with mobility and start collecting gold faster than most people. Remember teemo is a good farmer and you can normally last hit quite easily for even more gold farming just be sure not to starve your partner or they will die when you go to shroom

next return for Vampiric Scepter this item will enable you to keep in lane better and depending on your opponents stop your reliance on the health potions

boots of swiftness will help with your shrooming and keeping you alive remember teemos best form of defence is to never get hit and between exhaust, flash and your speed boost ability you have more than enough escape mechanisms

Zeal is where we start to get moving with teemo a little later than most but its worth the wait the speed boost is always welcome as well as the attack speed which we really need

phague will help with your health issue and the damage and occasional slow is welcome but not that reliable with the slow attack speed

Youmuu's Ghostblade is one of your core items and where your build starts to take off the passive is nice and the active I use as if it were an ability, changing my finger placement to
q w 4 t
(Dart) (Run) (Ghostblade) (exhaust)
you should now start thinking of theses as your 4 ability's with the bonus of r to drop mushrooms

Trinity Force will finish off the map control part of the teemo build with this insane speed you should be dodging all skill shots and planting mushrooms everywhere.

This is the point where the build starts to hold its own and you must ensure your team can carry you to this point. If you have a good team the map control you provide to this point should be more than enough to compensate for the lack of damage, and now its time to start dealing out some serious damage.

from here we need to sort out the carry part of your build by finishing off The Bloodthirstier for some serious damage increase as well as the Sword of Devine for a massive speed increase on your shots it also has a handy active which increases armour pen and negates dodge, I normally put this on button 3 using my w finger to activate it

on the occasions that the build get this far i either go for The Black Cleaver or Mandred's Bloodrazor depending on the team make up both will top up your speed of attacks and damage

with this build you should have max attack speed when you come out from hiding and use your Ghost Blade, to keep this more constant keep drinking Agility potions

Eve and other stealthers
these characters can really ruin your map control any good eve with get boots of mobility early and so you will be a sitting duck early game for them and a fed eve is still going to be a issue for you end game my advise is pick up a Quick Silver Sash early and this combined with flash should save you.

Karthus
his ult will half health you depending how well he is doing this can be a real issue for you, the best way to counter this is to buy Zhonya's hour glass and save the active for when he ults this is also a good item for you as it will increase the damage you do.

if your playing teemo AD or AP the point of teemo is still to shroom and shroom the whole map.

this will provide a few things
1) map control so your team can gank and avoid being ganked
2) ability to starve opposing team of buffs
3) the odd kill or assist due to people walking over shrooms
4) a handy escape trick for anyone near a shroom (hence they should be everywhere)

there are several types of shroom and each has its place
1) ward shroom used to show locations of the map (these mushrooms should be placed as not to be triggered but to reviel key locations such as barron)
2) perimeter shrooms used to protect people from ganks by ensuring the ganker walks over the shroom with time for them to escape the prime example is the bushes either side of mid
3) battle mushroom these should be dropped during battle to ensure you can fall back when needed and to catch out those who overextend (you should also drop perimeter shrooms during battle on all side passages when possible to avoid side swipes)
4) Defence field these should be placed when a turret or inhibitor is at risk. you need to place a lot of shroom at the location and this takes time, so ensure your using it wisely by farming as much as your can during the cooldowns (i managed to make a 12 strong shroom field after our inhibitor got exposed, which after our team got aced the shrooms forced the remaining 2 attackers to recall back to base saving the inhibitor, remember shroom fields take a lot of time to setup and are of limited use in a AD build but they can still be useful)

On reaching level 6 i begin to shroom my surrounding area and on my next visit to the shop i tend to go shroom the middle or the other lane and then mid if they are having a hard time. depending how the fights are going at around the 20-25 mid mark you will want to shroom up barron. if your shrooming well then there should never be a moment of where is the other team they should be running around and over your shrooms.

Below is a link to a Good AP Teemo Guide with a handy shrooming map
http://www.mobafire.com/league-of-legends/build/teemo-ap-dont-pop-that-24191
personally i prefer to double shroom barron, dragon and middle
with the dragon and barron this will make it so they wont back off and you can go jump them as a mushroom each side will rarely be triggered and with 2 in the bushes in each side of mid evenly spaced will give more protection for the middle lane player

the skill sequence above is rather flexible here are the rules i follow

1) always level up shroom when possible
2) get a point in run before 6 but then ignore it till the end unless your getting out ran
3) if your doing well at harassing get more poison
4) if you require damage prevention then get dart

Poison and Dart should be levelled depending on your opponents the above is just a rough guide

Flash: because it saves your furry little bum so much
Exhaust: your damn squishy and need protection while you unload on them

Masteries 9 0 21
i go for a utility build so i can make the most out of Flash and Exhaust and also keep my speed and in turn survivability up (you get over 500 movement at end of build)
the 9 in Offence will help max your damage output

Questionable Choices
Good Hands - well gota take somit and other 2 are pointless with this setup
Greed - not a game changer but handy little boost
Utility Mastery - i tend to save the buffs for other people if you want the buffs take a point from greed into here

Mark Desolation : to remove armour combined with you other 2 armour removing items you should hit all opponents hard

Seal Clarity : its not often that you will have mana problems unless you spam ur skills this set of seals should keep you going

Glyph Clerity : because cooldowns can save your life

Quints : Swiftness because a slow teemo is a dead teemo

1) if you get hurt in a team fight back up and if given the opportunity go life steal off wolfs etc late game you can restore a large quantity of your health and rejoin the battle

2) change your finger placement so you can use all your ability's and items, this build relies on the items actives heavily and you should be using them a lot

3) keep moving, you should be impossible to hit with any skill shot so keep your wits about you, as if you get stunned your pretty much dead

4) make sure flash is off cooldown if your going to places where you havent shroomed the surrounding area, assuming you get flash off you will survive most situations due to your ability move quick and the ghost blade boosts i tend to use them in turn so you can use the ability twice enabling you to outrun most people who are chasing even if they use ghost. and remember you have exhaust if there still following.

5) teemo can backdoor very well during late game just run up the jungle shrooming behind you and wait for the best moment to jump the turret. you can just about solo tank a turret but this will leave you very weakened so try get a minion wave with you. you should be able to plan your escape route due to the shrooms giving you map control just remember run like hell and keep flash handy and depending on the opposing team you can just stand still and hide whilst the danger passes. if possible whilst the team is tripping over all your shrooms in one jungle goto the other and regen your health on a wolf camp etc and go pop the other turret you will not have long to do this before needing to flee

AD teemo is a great character but you do need a team that requires a map controlling carry, if you do not have a team that can protect you then you will have a hard time with this build any one who can stun and give you a few seconds to let loose is a god send if not you will need to stay at the edge of the fight shrooming and harassing where possible

REMEMBER TO USE YOUR ITEMS ABILITY'S!! they make this template