Sign up for services like TuneCore and CD Baby. Submit your music tracks to them. These companies will upload your tracks to services like iTunes, Rhapsody, eMusic, and Amazon Music.
Sell download cards at your performances. Download cards contain a code fans use to download tracks online. Contact a service like Dropcards or Digstation and send them your tracks.
Build a website or pay a Web developer to build a site for you. Attach an e-commerce store to your website via a service like Easybe, E-Junkie, or Payloadz.
Promote your music via social networking sites like Facebook and MySpace as well as independent music sites like PureVolume, GarageBand, and Internet music forums.
Offer free downloads of some of your tracks. If a listener likes your music, he or she will buy your other music tracks.