This is my first guide, I'll try and explain a little as i play with teemo, I took some things from other places , but then I'll try to put my own explanations

sorry for my bad English

Camouflage
If Teemo stands still and takes no actions for 3 seconds, he becomes stealthed indefinitely. After leaving stealth, Teemo gains the Element of Surprise, increasing his attack speed by 40% for 3 seconds.
Blinding Dart
Obscures an enemy's vision with a powerful venom, dealing damage to the target unit and causing all attacks to miss for the duration.
Move Quick
Teemo scampers around, passively increasing his movement speed until he is struck by an enemy champion or turret. Teemo can sprint to gain bonus movement speed that isn't stopped by being struck for a short time.

Toxic Shot
Each of Teemo's attacks will poison the target, dealing damage on impact and each second after for 4 seconds.

Noxious Trap
Teemo places an explosive poisonous trap using one of the mushrooms stored in his pack. If an enemy steps on the trap it will release a poisonous cloud, slowing enemies and damaging them over time.

Ghost - Because of Move Quick, I would say you will rarely ever need this. But if you find yourself having trouble running away from champions because Move Quick is disabled when you get hit, this can save your life. Not a bad choice at all, but personally I would stay away. It isn't very useful later in the game, much like Clarity.

Exhaust - Slowing an enemy champion and blinding them is great. Blinding someone can save your life while you run for the safety of your tower. It can also save a teammate in the same situation. Slowing them down also has the same effect. You can also slow a champion that you are trying to kill so they stay within your range.

Buy Berserker's Greaves when you can.
Buy a Malady, this will make you attack a little faster, and debilitate your opponent's magic resists to further the damage from you abilities.
Buy a Nashor's Tooth this item gives you a lot of attack speed, AP, much needed CDR for your shrooms, and some mana regen.
Buy a Guinsoo's Rageblade this item is great for fights. It adds growing AP and attack speed, as well as the AD and AP.
Buy a Rylai's Crystal Scepter, this will make you a little more resilient and provide a spamable slow.
Buy a Rabadon's Deathcap, this will increase all of your other ability power gains greatly.

Pros
A great ganker with your passive and high movement speed.
A decent farmer.
With this build, your mushrooms will HURT.

Cons
Squishy.
Quickly targeted.
If you get locked down, you are in trouble.

Shrooming priority is colored with red being the most important or earliest to shroom and green being the last and least. This map is helpful no matter the side you are on.

More shrooming tips:

- Shroom placement is based on enemy movement and behavior. Think like your enemy. Know how champions work too.
- Shrooms are not an effective weapon in firefights. Instead, use your shrooms as a means of escape for your allies. Teemo is ranged. Avoid being caught in the fight.
- Players always round corners sharply. If you want a player to hit a shroom, place it close to the wall.
- In general playing, be aware of the map. This ties into the other ideas.
- When you can, disrupt the enemy's jungling. If you don't have a jungler, shroom your jungle as soon as possible although the enemy jungler will likely have taken your jungle long before you have your shrooms ready.
- Have plenty of cooldown reduction. Through no fault of your own, your teammates will always want more shrooms. The faster you stock them up, the better.

You are not the first to enter the fight and you cannot act as a good bait. "Ah hell Teemo is gonna lure us into a mushroom anyway".
If you have a good initiator you can pick off the enemy carries easily. Move Quicks active also allows you to run into a fight and drop one quick mushroom under one of your allies. Most people do not have the overview to see whats going on and the mushroom will explode turning more than one teamfight to your favor.

You can also create an escape route with one or two mushrooms and save your buddies from big bad autoattacks.

Tell your solo carry to take over your lane as a solo. You and someone else from your team go mid and you start fortifying it. You can hold mid while your carry gets some more farm and exp while being relatively safe and he might be able to push your lane and kill the turret.

Even if you have 10 shrooms at bot which are never going to explode doesn't mean they are wasted shrooms. They provided vision and an invisible zone of control so you could do your part in your lane.
Map awareness is not only knowing where everybody is but also knowing that nobody is at a certain location.