Ah, Teemo. Teemo, teemo, teemo. What once I thought to be an nonviable champion is now played by one of the highest ELO players (The Rain Man). Now, Teemo isn't a carry, nor is he a tank or support. In fact, there is really no case in which teemo fits into any team comp. This isn't ever a bad thing, it's just, let's say, Teemo can work with every team comp, because, well, he's just there.

Pros:
Pushes very well
Very good map control
Easy farming
Has very good movement speed to escape
Cons:
Must rely on team to hold back 4v5's or win 4v4's
Is not considered a real carry
Top lane is a must on Teemo to be able to maximize his pushing ability.

There's really not that much to say about the runes. I only recommend the ones that I go, but for as far as Glyphs go I recommend flat magic resist as well, I just enjoy the out scaling at level six, but hey, that's just me.

Marks: Armor Pen is great, AD runes not recommended.
Seals: Mana regen per level is so nice, I love it because you can keep using your shrooms whenever they pop up without worrying about mana.
Glyphs: Magic resist to keep chracters like leblanc and annie from raping you.
Quints: 2 Health for the early game squishiness, 1 Armor Pen for, well, armor pen.

0/16/14
WHY: Trust me, Teemo doesn't need more damage, because when it hits late game, all his damage is magic proc. and half of the time you won't even be in team fights.

Defensive: This mastery tree is commonly misunderstood in which some people feel only tanks use it. So why is it that I have 16 in defense for Teemo. Look at the stats I put them in. They are so, so very good. First of all, 48 bonus health is so nice on teemo with the health quints, and SoS is broken.

Utility: Must I say anything? This is very important on teemo. Increased base movement, flash cdr, and more mana regen with of course the 5% bonus experience.

Doran's Blade: More Health, More everything.
x2: Just trust me on this one.
Boots: Of course
Malady: Usually I hate this item on any champion, but teemo just makes it work because the damage stacks with his E, and it really boosts up his damage.
Madred's Bloodrazor: You're pushing, get a pushing item. Also, the 4% is great when your tanky and can live long enough to stomp tanks.
Frozen Mallet: "Hey, let's make teemo more annoying with a slow on every hit and MORE HP!"
Black Cleaver: Proc. is nice and you'll love the damage.
Guardian Angel: Self assurance that you don't die. I like that feeling.

Start with Doran's blade, try to farm as much as possible, and when you get back, try to aim for another doran's blade, boots, and madred's razor. If you can't fulfill that just get two doran's blades and boots, or just boots and more health pots. After that try to get a malady, this item boosts up your pushing ability and a bit more damage. Then it's time to finish off the boots. The reason why I get boots this late is because his passive on his W is ridiculous and you really don't need more MS til' later. After a bunch of farming, upgrade the razors to a bloodrazor. At this point in the game, you should have at least pushed top tower, and team fights may come up more often, so get a FMallet and BCleaver. Both have a very good proc. and are very useful on teemo. FMallet for the slow and health, and Cleaver for the armor proc. and more attack damage. At this point in the game, if everything's going well, there really aren't any more magic. proc items to get, so I suggest more defense. Guardian angel is a great pick on teemo just for the sight of being more annoying like "Ugh I have to kill him twice" kind of thing.

This is very self-explanatory.

Toxic shot and Move quick is so much better than maxing Q on AD Teemo. W makes up for tier two boots and E gives teemo ridiculous DoT and easier farm. Q is only there for the disable, which is still great, but you don't need more on it than 1 for early/mid game. With prioritizing E you get amazing auto-attacks, which I prefer much more than more Base magic damage on the blinding dart.

As I implied earlier, teemo isn't a champion that fits into any team comp. He really just sits at top and pushes like hell while warding the entire map.

At this point in the game, you should be taking the solo at top (implying there is a jungler on your team, if there isn't just duo lane.) Middle isn't a very good choice for teemo because he pushes, and mid-lane is always in conflict later on, where as top lane can be pushed to your delight. Really at this point in the game you should just focus on last-hitting the creeps, and if someone wants to trade hits with you, go for it, just Q + auto-attack and its already 2-3 bars off your opponent. Pretty much just farm until you can go back with doran's blade, razors, and boots, the essential early-mid game items on Teemo.

At this point you should be around level 6. Now it gets to the fun part, the mushies! Oh man mushrooms are so fun to plant. But don't get the wrong idea. Mushrooms ARE NOT THERE FOR DAMAGE. They're there for map control. Even Rain Man places his mushies in places people don't walk near so he can have sight for the full ten minutes. Here are some good ward spots for Teemo.

The spots circled red are the places where you should always keep a shroom. Where as the other spots are just good places when you can get there. Always try to use your shroom when you have all three in your bag, don't want to waste all that cooldown time. Sorry if the images aren't exact but, try to put the shrooms in places where people won't walk over it. Once again, its just there for wards, not damage.

At this point in the game, you should have finished your build or is getting to it. And if the game has dragged out this long, you either have not done your job right or you've been doing it right and your team can't survive. Now its just all about how you play. Your next fights are crucial so I recommend being in them. Let fate take hold of the rest.

PUSH PUSH PUSH PUSH PUSH PUSH PUSH PUSH PUSH PUSH

Teemo is a great champ and you should really try picking him up.
He's fun and viable, just not the typical viable.
Great in ranked solo queue, great in ranked games. Great champ overall.