Start with a dorans´s Blade because its good and give 100 hp and 10 AD + 3% Life Steal
Then i go for mercury Treeds for the tenacy Its allways good. Then you buy another dorans´ blade.
Now you gonna buy 2 daggers for the malady becuase this item give (as/ap) Good hybrid item.
And buy a stinger because you need a nashor´s Tooth.
Then you need a frozen mallet for the slow and some health.
Now you sell the dorans blade and buy a warmong´s armor.
Then you buy a wits end or a atmas.

I Take Flash because i just good at all, and its good to escape. And ignite because its good to finish the enemy off, and good vs supports or tanks.You can also take Ghost so you can use move quick and ghost on same time and get away in no time. Exhauts is also a good summoner spell you can use it to slow the enemy or use it on a ad champ so he cant do any damage. And Clairvoyance is a good support spell if you wanna play teemo as a support. If you wanna play him as carry you can use cleanse.

I take Defence masteries becuase teemo only got hes w to escape and he dont need full offence masteries. Because teemo is not the most tanki champ in lol so he need tanki masteries.

When You Are at 5:00-15:00 Mins cap. You need to harass just poke to the champs with you Toxic Shot and try to get toxic shot on every minion if you not good to last hit but if you good to last hit you just need to harass while you are farming. And use you Ulti as a ward so place them in the Brush And play carefull if there is a jungler teemo is not good to run from gangs. Teemo is a good solo champ he can solo top and go mid or even be a aura Support.

Teemo Is verry good to counter. Tryn, Master yi, Jax or just auto attackers, he is also good to counter ad carrys.

I take Flat armor Runes Because now teemo be one of the best vs auto attackers.
And i take Armor Pen because that very good to teemo so he can counter armor tanks.
Then i Take ap per level because teemo is not good with ap early game but he is very good late game.
I take movement speed because teemo can be use for the extra Moventspeed and with hes Move quick only rammus and yi can get him.

You need to use you blinding dart to save you team mates from attacks and also give a little damage in the background you need a little cs but not much. and you can use you move quick to get to the team fights and you can use you ulti to ward. And you need to use utility masteries Because you need to spam you blinding dart.

To Runes You need AS Runes so you can deal a little damage and you can spam toxic shot
and you need gold per sec so you can get gold even if you dont farm
You also need some mana regn Per level
And You also need some healh regn because teemo cant heal him self You only need this in the second Build