The tittle says it all...

I personally pick flat AS/AP/AD runes because Teemo is a SnowBall champion (Snowball champ means that if he does good in early game he'll be very good afterwards).

30/0/0 LIKE A BAWS
AP/AD Masteries with a little bit of CDR and AS.

Firstly pick the AP Tome whose name I do not remember... and some helth pots if you want. I personally don't
Afterwards start building Berseker's Greaves and continue with Sheen.
Buy a Fiendish Codex and then Morello's Evil Tome.
Take Lich Bane if the enemy team has alot of AP and then pick a Pickaxe(eeh eeh get it?? you don't? ok...) for your Guinsoo's RageBlade.
Take something that helps you in the exact game like a Frozen Mallet if you want health. Nashor's Tooth if you want Some CDR/AS/AP.
Wit's End if they are AP.
Madred's BloodRazor if they are Tanky.
You could even get a ThornMail if you are gatting too much damage.
Continue with your Rabadon's and you'll be pwning some a$$.

I take Toxic Shot for its on hit effects and the poison or as my friends call it Dat Poison...
take Blind for the squishy AD Carries (Yi,Xin,Brolaf,Fiora, etc etc etc)
Then Move Quick to move ya know ... quicker...
Max Toxic first
Then Blind
Max Move Quick last
Mushrooms(Ult) are,like usual,picked at 6 , 11 and maxed at 16
Teemo has a pretty cool passive.
Try and stay stealthed next to minion fights for the XP and UnStealth only to Secure the Champion Kills.
When you do that type Kill Secured or KS on the Chat so everybody will know that your teammates couldn't have done it without you.

Like Phreak once said "I play Teemo as a ward"

I use Flash and Heal
Many say that "heal is gay... blah blah blah Morderkaiser est #1 HuEHuEHuEHuEHuEHuEHuEHuEHuEHuEHuE"
and other BullSh*t...
Heal can realy help you stay alive in your lane and level up fast in early game.
Flash is for the gataways but try to TrickFlash(I know that i just made that up but please try to flash trough walls and these kind of stuff).
Other choices
Ghost: to gang faster or for escapes.But you have W for that so i wouldn't suggest it.
Ignite: to last hit the champions which can be replaced with Dat Poison.
A really good idea is Surge. Even though you're gonna laugh Surge is really good for Teemo because it increases AS and AP.

I hope you liked my Build for Teemo *Cool music playin' on the background...* The Alive Ward.
I'm really sorry for my bad English... Please vote and comment.
Have A nice Day.