I just found out about this online guide to iMovie ’09. It looks really good. I strongly recommend it. It’s long, and I know you’ll find that actually using iMovie is less complicated than reading about it. But if you find that the videos I’ve linked to in the syllabus leave you flummoxed, please take a look at this valuable free resource.
Among other things, there’s a nice explanation of how to insert a cutaway, which is another term for B-roll.