Runes, runes runes. I often see magic resist or something like that one Teemo as the runes section. I personally do not think they help that much, due to first blood being auto attacks and abilities become main part of mid-late game making your character still squishy if you don't also invest in some sort of magic resist or armor.

But i go with:
-Greater Marks of Insight(Level 3 Marks of Magic Pen.)
-Helps auto attacks early-late game as if following this build it is the ONLY form of Magic pen.
-Greater Seal of Alacricity(Level 3 Seals of Attack Speed)
-Helps w/ hitting faster as teemo is AP/AS hybrid.
-Greater Glyphs of Potency(Level 3 Glyphs of Abilitiy Power)
-Helps Teemo's passive attacks early game-mid game. Also helps when Rabadons come into play.
-Greater Quints of Insight(Level 3 Quints of Magic Pen.)
-Same as Marks.

I go 9-0-21, due to I want Teemo to invest points into his Spells, as well as plant mushrooms faster. I go for offense for the Spell Pen. This greatly helps Teemo's attacks. Also, when you out level somebody, you have a great advantage. So I like to get Awareness(+5% Xp).

First I'd like to apologize if the skills levels are wrong. Basically, 1=Toxic 2=Speed 3=Toxic 4=Blind 5=Toxic 6=Mushrooms. You get the idea right? Level up toxic and then speed and then mushrooms when available. Oh, but before you try this, please, make sure you are lucky.

The main point of getting your Toxic first is for damage. This guide is ALL about AP damage and destruction!! Mwuahhahaha. xD

The item set leaves you squishy, so you'll need to be the last to enter a team fight, perform ganks. And stay passive, except for farming minions, enemy jungle, and helping push or kill minions while harrassing enemies in laning phase.

The Boots help w/ Teemo's escape, as well as moving around the map. Pure movement speed in boots are completely pointless. So to get your money's worth go for either Sorcerers, or Beserkers.

The next item, Malady helps shed the enemies hit points. If you are an increadible Teemo you can dodge and use these items to that maximum potention. This build will make your attacks hard.

I then buy Wit's End, which also helps stack upon the damage with Malady, while boosting your attack speed.

Next I buy Madred's Bloodrazor which stacks w/ the previous two items in damage and lets your destroy a tanks health. Then when possible get Red Buff. These three items, with a red buff=crazy damage.

Next I buy Frozen Mallet, as people will fear your and run away. Plus it boosts ur small squishy health.

And finally, the icing on the cake. Rabadons, this greatly helps ur toxic shot, which is passive. This also creates a small bang w/ your shrooms.

I use Ghost and Flash, basically because this build is squishy, so you'll need some escape ability. These are self explanitory.

Pros:
-You'll get amazing damage
-Amazing Escapes
-Shrooms for wards
Cons:
-Squishy
-Need to be last into team fights, so your not targeted first
-Need to be lucky w/ escapes

-High Damage
-Squishy
-Last to enter team fights
-Lay shrooms like a chicken on crack
-Farm enemy jungle(when towers are destroyed
-Own that red buff!! (not at lvl 1-5 tho)
-Be a Ninja!! or at least a pain in the butt
-Cover your escape w/ shrooms

Best of luck to you w/ my guide!!