Public service announcements are free advertising, run by the media as a public service. They most commonly appear on radio, sometimes on television, and once in a while in print media. PSAs also help fill up unsold advertising slots.
Radio Let's start with radio.
Every radio station will have someone in charge of PSAs. Call your local stations, learn who the PSA person is, and find out what the station's PSA guidelines are. All future communications with the station should be aimed at the PSA person. Read the PSA out loud to yourself many times, so that you can revise it and make it as close to perfect as you can get. Public service announcements have to relate to some type of public service.