Hello, my English isn't the best but I think that you'll understand what I'll be trying to say/write. I dont really want to talk to much about teemo in this section, because everybody know him. I like this char.

Pros:
-Very fast, in early game too.
-His posion can help to make last hits.
-Blind is a ****ing good spell, which makes him unbeatable in 1v1 against other ad carries.
-Shrooms are very funny in good place and good time ^^
-With those items he can make a huge amount of damage in a few secs.
-His passive is good for ganking. (or hide)

Cons:
-Squishy in early game, and later too.
-Almost the most focused character.
-Can hardly escape from unshroomed areas.

I dont think that it should be explained. You need huge attack damage but from step to step. So buy those runes which can make u stronger by levelling up. That's the reason why i chose Greater Mark of Might and Greater Quintessence of Might and Greater Seal of Strenght.
Teemo needs health regen at early game so i put 5 Greater Seal of Regeneration.
And for glyph i chose magic resist but if u want to have other runes instead of these it is okay.

Teemo needs taking huge amount of damage and u can reach it with Brute Force, Deadliness and Havoc. Attack speed and armor penetration is obviously needed. Summoner's wrath and Summoner's Insight is needed coz of ignite and flash.

Spells. I strongly advise Flash because it's a good attack and retreate spell and it can help u to get out from hard situations. Ignite is also a perfect spell for teemo, with teemo's posion and his mushroom and ignite it'll be a huge amount of DAMAGE! :) and it's also the best at late game. Better than exhaust because your enemy cannot attack u while the blind spell is on ur enemy and in late game every ad carry and other type of char (expect for tanks) will have life steal or spell wamp and it won't heal that amount that they are expecting.

Teleport: not absolutely useless, because u can teleport to ur shrooms too!
Heal: It's kind of useless in late game. U can choose it but I think flash+ignite is better.
Ghost: U may can choose it instead of flash, because ghost plus Teemo's W is a good combo for attacking enemies and running away from them.

First buy boots of speed to ur W spell and 3 health potions because it's always a good decision. Buy your boots totally (I mean the Berserker Graves)- then buy malady but first the 2 daggers because u need attack speed. Then buy malady at least.
If u finished malady u can choose: buy vampiric scepter or the recurve bow. If u bought the bow I think it's the best that u finish wit's end. If u got it u should buy vampiric scepter. If u reached this status u'll get 1.8-2 attack speed and a big amount of damage because of malady's and the wit's end's unique passive spells.
Buy Frozen mallet. U need to buy it but not to slow enemies. It'll give u some health.
At least u can buy Bloodthirster.

As u can see the sixth object is missing. U may can buy a Madred's Bloodrazor because it'll give u a better magic damage (: or infinity edge which will give u a very good critical strike rate and u'll make 250% crits. If u bought Infinity edge, U can sell one of ur weapons (malady or wit's end) and buy a madred's bloodrazor instead of it. Because it'll make a bigger damage (:

Early Game and Mid game
Always buy some health potion when u get home. it's very important in early game! Always harass! If u can shoot just 1 shot, do it! It should make a bigger damage than u are expecting because of ur posion. Teemo is usually a bot charachter but he is also good at solo top, but only if it'S 1v1 and the enemy is a melee. It will suck coz of teemo's posion and early harass. After u reached lvl 6 u'll get ur ultimate and the fun is just have started. Put shrooms at the edge of every bush and put in the middle of the way. But u need to see where the minions going and u'll put the shrooms out of their area. It's a good idea because nobody will think that u put a shroom in his way if he/she isnt going in bush.
Stay at ur lane and try to push as far as u can. After finishing the outer turet go and shroom everything not just bottom and help ganking to ur jungler or without it. But if u dont gank it's not a problem, just farm the minions. Make sure that u put a shroom in front of baron.

Late Game
The focus will be on u. So try to take the appropriate distance between u and ur teammates. If u can do it u'll make that damage that they want. If u and ur teammates are killing Baron makes sure that u have put shrooms in the way of ur enemies. It's always important.
If u and ur team is on loosing, teemo is a good char to backdoor. Just bring a 5 sight ward (if there aren't minions) with urself and put down one by one and destroy the turet. Always put shrooms in ur escaping way to home. If they try to catch u they'll bump in ur shrooms and it's enough to press a W and go away. If it is complicated stop and stay in a bush til u turn into invisible.

Teemo is a good character and really really easy to play. I hope that u enjoyed it (: