Have your recorded song mastered. Mastering your song will optimize the volume, dynamics and EQ, thus making the song as professional sounding as possible. Have it done professionally or on your own.
Assign a UPC (Universal Product Code) to release your song for sale, as well as an ISRC (International Standard Recording Code). Obtaining an ISRC is free most of the time, yet some music services charge a fee for a UPC.
Set up a pricing plan for your music. Depending on your music's popularity, you will want to set up a different pricing option. For instance, if your music is not well-known, use a commission-based pricing option. Get a flat-fee structure if you expect your music to sell well.
Use an online music service that will digitally distribute your music to iTunes and other sellers, such as Rhapsody, Napster, eMusic, etc. Several of these music services are listed in Resources, varying in price.