Okay so its hard to explain without pictures. But essentially you import your video clips into imovie (camera attached to computer there should be a button with a camera in the left center of the window) Then drag what ever clip you want to use into your project space (upper left corner)
When you hold your mouse over the clip, a little button should appear in the bottom left corner that looks like a gear. Click on that, then go to audio adjustments and drag the volume bar down to 0%
That's to take the sound out, then to ad music you just click on the musical note button in the right center, choose a song and drag into your project.
The more you play with it the more your figure it out. I used imovie for a long time before buying final cut express, so feel free to ask any questions.