There are several ways to do this.
1) One way is to drag the music track in from the Music browser, and drop it on the frame where you want it to start. The music track shows is a ribbon beneath the video clips. You can drag the left edge of the music clip to the right to shorten the beginning. You can drag the right edge of the music clip to the left to shorten the ending.
2) You can drag in the music track as before. Then click on the small gear icon at the left of the music track. You can open the Clip Trimmer and trim the music track there.
Here is help for the Clip Trimmer.
http://help.apple.com/imovie/#mov3b0fa95d
I recommend that you watch this video tutorial from iMovie 08 days for how audio works. There are some new capabilities in iMovie 11, but the basics are the same.
http://www.apple.com/findouthow/movies/imovie08.html#audioclips