Contact a service like TuneCore or CD Baby. Submit your MP3 files and artwork. These services will make your music available digitally on online music sites like iTunes, Amazon, Rhapsody, and eMusic.
Sign up for accounts at sites like PureVolume and GarageBand. These sites allow you to post your MP3 files and include links to sites like iTunes and Amazon so people can buy your music.
Contact a web developer and have him develop an e-commerce store on your website.
Contact a commerce site like E-Junkie or PayLoadz, which will direct customers to an external site off your website.
Contact a bolt-on service like Easybe, which will set up a record store attached to your website.
Promote your music. Post links to your music in forums and blogs, and on social networking sites like Facebook and Myspace.
Sell download cards at your shows. These cards allow people who buy them to go home and download your music.