How do I wipe my MB Air's drive and start fresh?

I want to return my Air to the original factory settings, to the way it was before I added all kinds of junk to the solid state drive.

I'm running Snow Leopard. So after I wipe it clean, I plan to go to the App Store and buy Lion.

If you bought a pre-Lion Macbook Air, it should have shipped with a system restore USB drive. This is exactly the scenario for which it exists: restoring your system to factory condition. Back up all your data, so you can keep what you want. Then, plug in the restore drive, and hold down the option key (⌥) when you boot. Select the restore drive, and follow the instructions to install a clean install of Snow Leopard.

If you're planning on immediately upgrading to Lion, you could save some time by downloading Lion first, and making your own USB installer. You can still wipe the drive clean first, but this way you're only doing 1 OS install instead of 2.