OK so this is my first build, please leave comments and suggestions below :D.
Alot of people like to use AP Teemo, because his shrooms do a ****load of damage. True, but I find he relies on them too much if he's AP. His attack speed is low and so is his damage, and I just personally prefer to play him with this build.

For runes, I get the greater mark of alacrity and Greater quintessence of Alacrity because it's always nice to start off with some attack speed, and helps send the opponents back to base quicker obviously. I get Greater seal of evasion because it helps with survivability and it helps quite a bit with the 6.5% dodge. Finally, I get the greater glyph of replenishment so you can stay in lane longer, out-level your enemies, and win :D. You shouldn't need the mana regen too severely, as leveling toxic shot a few times early-game really grants good harassing.

I choose the 21-0-9 mastery set because the critical strike is always nice and as you already have your phantom dancer, you will be getting crits very often. Exhaust should be obvious. Attack speed because i just love it <3. On to the utilities. Seeing as I use flash and exhaust for my summoner spells, I take mana and hp regen with a little less time dead, with bonus exp and more mana regen.

So i try to rush Phantom dancer as it's like your base item for everything. Gives you more attack speed, crit, and move speed, all things that teemo requires IMO. Frozen mallet because other than exhaust, and your speed, you have no slow, stun or anything else that could help in chasing situations. Also pretty much removes your squishiness with the huge HP bonus. I then go for bloodthirster because without it, you can still be pretty easy to kill. Also, who minds 100 extra damage right?When you get 10% additional lifesteal with it, the other team will stop targeting you first 100% of the time and will go for someone else sometimes. Bloodrazor for the attack speed/damage. You could swap this for a black cleaver, exchanging that bit of attack speed for more damage, they both have pretty good on-hit effects, so whatever floats your boat. Finally banshee's. This is because often late-game, people will still be targeting you, as teemo is not exactly a tank :D. This will block the initial stun or slow, and allow you to run away with Move Quick unscathed.

I take blinding dart first, as it will help whenever there's a big clash. Toxic shot can't do much at rank 1, and blinding dart gives a blind for a couple seconds, which often helps me get first blood. I level toxic shot twice at level 2-3, to start harassing effectively, and move quick at level 5. Level your shrooms at level 6,11, and 16, as is normal with every champ. Get Blinding dart to level 4, as it starts to do some serious damage, get 1 more rank in toxic shot, and then max them both out, with blinding dart first. I only get Move quick last, as you have pretty damn high move speed with the phantom dancer and rank 1 of it.

I get flash, as it helps a ton getting away from sticky situations and for getting that last hit on someone who's running from you, and somehow managing to get away. I also get exhaust because, as I said earlier, you have no other form of stun or slow except for your frozen mallet, but you get that around mid to late game, so it helps with early ganks.

Well as I said in the Intro, this is my first build, please leave tips and comments, don't start trolling or ill eat you :D.