A nice Teemo build I tend to go for, it gives good amount of AD and AP for both sides of Teemo. Mushrooms will do high amounts of damage and you will also be able to hit the enemy quite hard and drop squishys very easily with high attack speed.

Greater Mark of Desolation Helps Teemos attacks get through more heavily armoured characters.
Greater Seal of Alacrity A good boost to attack speed which is one of the corner stones of this build.
Greater Glyph of Potency Boosts up Teemos AP which increases the damage of blinding dart, toxic shot and mushrooms - Always good to have these doing extra damage.
Greater Quintessence of FortitudeGoes without saying - Teemo is VERY squishy, extra health is a plus.

Some possible alternatives

Attack damage marks are a possibility, but later in the game 10 AD doesn't matter that much especially if you get to the Rageblade and Beyond.

hextech GunbladeA very good item, life steal and spell vamp along with AD and AP means it is a greta item for a hybrid Teemo the life steal and spell vamp can help to keep your low morale up whilst the high boosts to AP and AD are awesome for taking down enemy champs.

MaladyA good boost to attack speed and Ability power, the passive is also nice for reducing enemy resists and dealing that extra bit of damage.

Guinsoo's RagebladeOnce again a great hybrid item, AP and AD along with a great passive which can increae attack speed by 32% and AP by 48, if you play smart with mushrooms you will be able to have this up a lot in team fights dealing a lot more damage.

Berserker's Greaves I choose these because the 25% attack speed and 2 movement speed are great for Teemo. Boots can be changed however and Mercury Treads are useful if you see the enemy has champions such as Jax and Sion, the 35% reduced duration of stuns can save you. Boots of Swiftness are an option too, but with the passive 'Move Quick' movement speed is generally not an issue.

Nashor's ToothSimilar to Malady it provides a massive boost to attack speed, Ability power and reduces cooldowns by 25%, allowing you to put more blinding darts on auto attack heavy champs such as Ashe which hurts them badly.

BloodthirsterThe last item in the build - this one is very much interchangable. There are numerous other items which you could use instead. Infinity Edge, Rabadon's Deathcap, Black cleaver to name a few. It's worth noting that the game probably won't go on long enough for you to get the final item.

Build Cost = 14,700. Overall the build isn't too expensive and does a lot. Infact the cost would go down if you went for a black cleaver, and wouldn't increase by that much if you went deathcap/infinity edge.

I use Clarity because Teemo is a great support character and also has high mana costs - this helps keep not only Teemos mana high but also your lane partners if you aren't soloing.

Teleport - I use this because it allows you to move around the map quickly and suprise enemies. Teemo has a good burst of Blinding Dart allowing him to burn down the enemy significantly before they can do any real damage. (I wouldn't reccomend trying to gank characters such as Malzahar - they will simply eat you.)

A simple 21-0-9. 21 points in offense to keep Teemo's damage high and 9 in Utility to boost Teemo's support even more. Masteries are flexible and could be changed at any time 9-21-0 can be a good build to go for as Teemo is squishy.

Blinding Dart > A lot of auto's > blinding Dart. Pretty simple really, Teemo only has 1 combat skill per say. Toxic shot applies on auto attacks.

MUSHROOMS This is Teemo's best attribute, his ability to place mushrooms around the map. While laning you can place them effectively to cover ANY entrance to your lane, and in the bushes. This will keep your alive and make you easily spot ganks when they approach. It is also a good idea to have them at Baron and Dragon to make sure the enemy don't start farming up Gold/Buffs quickly.

Mushrooms are also a very effective escape tool, setting a trail as you approach to gank an enemy can prove very effective both for baiting them and easily taking them out with the high damage, or if something goes wrong, escaping. The massive slow and damage will stop them.
Stealth
Stealth. A very important skill for teemo. Upon taking no action or movement for 2 seconds Teemo stealths. In early game this can be used to effectively gank unwary champions, late game it is also very useful too as it allows Teemo to wait and ambush the enemy. The attack speed boost for 3 seconds after leaving stealth is great too.

Mushrooms are a very effective tool at this. You will be able to wipe out 10+ creeps at once with a single mushroom, run in, place it and lure them over the top. This is one way in which the item build is good, you are able to get items very quickly with good farming.

Lizard BuffTry to get this as much as possible, the slow and DOT means enemies hardly ever get away, combined with the DOT from toxic shot it's very effective at stopping runners.

Golem BuffIf it's available go for it, the mana regen is quite nice for spamming shrooms and blinding dart. the CD reduction helps too.

With Teemo play safe and use stealth and Mushrooms to their maximum potential. Laning can be fairly easy with Teemo, both solo and with another player.