Teemo is a very fun, annoying, and flexible champ. Teemo has some great advantages, and some crippling flaws. Teemo's laning phase is great, his range is a good, and he is targeted instantly. When he pushes the enemy out of lane, he can quickly get down to mid lane, blind the enemy AP Carry and get a few Toxic Shots in before they run away, but don't forget to help mid lane out with shrooms. If they don't have active wards, put the shrooms in the area's that are most susceptible for incoming gankers. If they have wards up, place them on the sides of mid lane so that the minions don't hit them, but the enemy champ going around the minions for better positioning do. Teemo's shrooms act as tiny tiny wards.

I like to place 1 or 2 shrooms in the lake where enemies HAVE to walk through to gank you (put the shrooms in the skinniest area of the lake so they are unavoidable. Inbetween the Baron, tri-bush, long-bush, and Blue Team's Blue Buff enterance), then one in the cebter of tri-bush. Extra shrooms should be placed in your lane around the minions, (the last thing you want is to waste a good shroom on those minions) so that the enemy champ triggers them. The shrooms act as small wards, but they last for 10 minutes and need to be replaced when the old ones expire or are triggered.

Teemo can carry 3 shrooms at once and gets a new one every 20 seconds or so, this is affected by CDR, so the Morellonomicon gives you shrooms faster. Teemo pokes and re-positions to poke again, u don't burst your enemy, so poison is your best friend. Also, if you see an unavoidable gank coming, dash into the bushes and stand still, 2 seconds later you become invisible. Even if they know you are still there, they don't know where in the bush you are. If they have skill shots or AoE abilities, they will try them, but don't panic, just sit there and take it until your chance to dash out is ready, or they give up and continue farming. Also, because you are fast with Move Quick, when running from an enemy, don't be afraid to drop a shroom in front of you and keep running to slow them. If you are being chased with no chance of escape, do your best to make them chase you into your team, to a turret, or just go all out and drop all your shrooms and use your Blinding Dart for damage and then poison them until you are dead, the damage will make them have to return to base before safely pushing again.

Teemo's biggest flaw is that he doesn't help teamfights a lot, this depends on how you use him in team fights. NEVER be one of the closest 2 people to the enemy team, poke with posion as often as possible and drop shrooms during all the commotion. Save your Blinding Dart for the AD Carry, and use it on them whenever it is off cooldown. Teemo's speed makes him a great backdoor champ. While the 2 teams fight, you go off to bottom or top lane and push until the enemy team notices you. Also, near the end of the game if your teammates have Teleport, they can Teleport to your shrooms, so place a few in the enemy jungle near their base.

Teemo's main item is Liandy's Torment. It makes Teemo's shrooms devastating, especially to tanks because it takes out percentage of health and it's doubled to enemies who are slowed, so his shrooms go hand and hand with Liandry's Torment.

Rylai's Crystal Scepter works well with Liandry's Torment because it slows for 1.5 seconds, which isn't helpful for his shrooms, but it makes Blinding Dart do massive amounts of damage. It makes Blinding Dart damage, burn for a percent of target's health, slows, and blinds the target. These 2 items make Teemo a handy little Yordle, and provide him with 1100 health, 130 AP, and 15 MP. That is quite devastating even without the passives.

Rabadon's Deathcap provides Blinding Dart, Toxic Shot, and Noxious Trap with great damage output. Sorceror's Shoes provide a little extra MP, and the speed required to chase and escape. Crystalline Flask is great for Teemo, it gives him the exact amount of sustain to stay in lane long enough do make a difference, and you don't need to buy health or mana pots, so it saved precious gold.

The Morellonomicon gives you CDR, so Teemo gets shrooms quicker, it also provides all the mana you will ever need, but the passive is what makes it so good. Grievous Wounds is applied to all enemies under 40% max health who are hurt by your magic damage. So, a weakened Volibear who accidentally triggers your shroom will start to take damage. When his passive kicks in to heal him, it won't help because Grievous Wounds is doing its job! Same goes for all healing champs, including that unkillable Dr. Mundo.

Guardian Angel is the defensive item you want for Teemo. Why? Because he is squishy, but when he dies, he comes back with about 880 health which is enough to pop that Zhonya's Hourglass, by the time it ends, if you still need to leave, Flash away and use Move Quick to try and escape.

Self explanatory, normal AP top lane. I'm not going to go into detail.

AP Quints for good early game damage.
AP Glyphs for a steady damage output with Toxic Shot.
Armor Seals so you can take a hit.
Magic Penetration Marks so you can counter a little of the enemy's Magic Resistance.

Toxic Shot first for harass and easier time last-hitting.
Blinding Dart next for those big meanies who try to get to close.
Move Quick after for some chase and escape.
Noxious Trap whenever you have an available point because this is Teemo's bread and butter.

Flash for escapes and securing kills, must have on 90% of champs, no explanation needed.
Teleport is the interesting spell because you can gank any lane very quickly and late game, you can use it to get to one of your shrooms. I didn't take Ignite because you have poison and mushrooms, but Ignite with poison can give you first blood which is nice. But it doesn't help too much later on.

Pros

Ranged
Good damage
Very effective traps
Fast
Has poison
Has a blind

Cons

Squishy
No Burst
Instantly targeted (which can be a good thing but isn't normally)
Can't kill an enemy alone unless they have been weakened already