Convert the songs to WAV files in stereo with a 44.1 khz sample rate and a 16 bit sample size.
Visit a site designed to edit and sell digital music. Some of the options are CD Baby, SongCast, The Orchard and TuneCore. Create an account on that site.
Use the directions on the site to upload music and vital information such as the name of the song and the artist. Add a graphic at least 1600 by 1600 in size and at least a 72 dpi resolution. The graphic is optional, but vital to attract attention to the music. The site will handle everything else to get the material on the retail sites.
Pay through PayPal or a credit card. It may take up to six weeks for the sites to list new music.