Also you will take 2 Greater Quintessence of Insight instead of 2 Greater Quintessence of Potency!
Armor and Magic resistance is good to have on Teemo since he is a really squishy hero!

Teemo is a really squishy hero thats why i take Heal(saved me a lot of times and my teammates!).Teemo is a kind of hero with which you should play hit and run, so taking Flash is a good thing to get out of nasty situation and combined with Move Quick you can easily gain on escaping enemies.

Teemo as said before is a hit and run hero. Don't forget that you are a squishy hero so avoid being in a middle of a teamfight, just stay on a side and spam Blinding Dart and auto attack!
His ulty is a very usefull thing both for anti-ganking and as a "ward". Learn where players mostly pass and put Noxious Trap there. As a "ward" it can be used near Baron,Dragon,Blue and Red buff so u can see when the enemy team attacks one of those and your team can gank them easily that way.And of course Noxious Trap it's a great thing to lead some melee heroes into it and just shot them while they are slowed.
Blinding shot is very usefull against auto-attack heroes like Olaf, Trynda, Ashe, Caitlyn, Graves... Also attack speed serves him well for Toxic Shot

-Doran's Ring is a good item for an early game. It gives you the much needed HP, 15 Abillity Power for your Blinding shot and some mana regeneration.
-Sorcerer's Shoes not much to say about this.Gives you the needed magic penetration.
-Nashor's Tooth gives you attack speed which is very usefull for Teemo's Toxic Shot and also gives AP and cooldown reduction for spaming.
-Morello's Evile Tome gives you a alot of Abillity Power and the needed mana regeneration and cooldown reduction.
-Rabadon's Deathcap gives a lot of Abillity Power.
-Rylai's Crystal Scepter gives you the needed HP,Abillity Power and is slowing down enemies which can be both used for escaping and killing enemy heroes.
-Abyssal Scepter Gives you Abillity Power and surrounding enemies lose their magic resistance.