Import your video footage so that it can be captured by the film-editing program. This is done by connecting your digital camcorder to your Mac and waiting for the dialogue to prompt you that you want to upload the video. Once the video has uploaded, you can begin editing scenes and pairing them with background music.
Choose background music that fits with the particular scene you are editing. iMovie has a few stock sounds/audio files available, but to increase your options, you should choose the background music you want from your iTunes library.
Click the "Media" button in iMovie and then choose the option for "Audio." Select the "Timeline" icon to get a list of the video's tracks. From this list, you can click on iTunes Library and decide which track you would like to use. Drag the playhead on the video timeline to where you want the song to be placed and then click "Place at Playhead." From here, you can adjust audio levels and shorten the length of the song to match the length of the clip.