This Teemo build is an example of how you can play the character in a "AD carry style", but dealing mostly AP damage. This is an exelent choice of pick for you who plays a lot of AD carry but needs to have a AP based champ. Beacuse of Teemos extrodenary function to fill the role, he can be played in both top and bot this way.
The runes focus lays on Magic Penetration, this is simply because it scales signiflicantly well with the AP% equips you buy to him.
Now in season 3 they have buffed all champions base movementspeed, therefore you can buy a Doran's Blade to start with, it simplifyes your early farming and you just get that extra HP that erase you from the "free kill radar".
Boots you buy thereafter, you decide which fits you the most, Berserkers for more DMG while Mobility gives you great roam capability. Boot enchantment is something that you also buy completly depending on your own playstyle.
After boots you buy Phage and Hunting Guise just because it gives you a good starter for your Core Items.
When then buying Core Items I would recomend to buy Hurricane first after you finish the "Core-Starters" just because you then will have it really easy to farm, and farm is extreamly important for this expensive build.
The rest of the Core Items will then give you very high DMG that mostly are percentual, which means that High HP tanks will melt down easy by you.
The last equips you buy have to fit the sitiuation of the game and your playstyle.
For example, if you want to shroom the whole map and wait for a Pentakill, you buy Rabadons.
FARM AS MUCH YOU CAN!
Always max the poison first, then choose between even further DMG or Mobility. If roaming alot I recomend W.