Learn what local radio station caters to regional singers and bands. Alternative rock and local public broadcasting stations often have at least one show per week dedicated to playing music written by local musicians, and college radio stations are often interested in taking chances on playing new performers that the larger stations may not have time for. Scan the stations on your radio and take notes of which ones have times or shows featuring local artists. Once you find such a station, listen for station identification between songs and use its call letters to find its web site.
Go to that station's web site and look for a link that says "local bands" or "local music" or something similar. For instance, in Portland, Ore. KNRK's web site offers a button in the bottom left corner of its web site that reads "Your window to local music." Clicking on that button provides you instructions on how to upload your MP3s.
Follow the instructions on the web site, fill out the required forms or information fields and upload your file.
Keep looking for more stations. The more you find that will accept your MP3 uploads, the more air time you are likely to get.