Ha, looking for imovie help myself when I found this. Not trying to deter anyone from the forum, I'm sure you'll get answers here too, but every time I launch Imovie I know I'm going to spend hours doing what should be simple. It's NOT a very intuitive program at all. But… try youtube. BE SURE you search for the specific version you have too. Enough changes that tutorials will be useless if your on a different version.
Then start fishing. Some are aweful and some are spot on. Believe it or not I found that kids can put together the best tutorials. Some of the real fancy professional looking ones blabber on and never answer the question.
Note; I was just searching yesterday for a different question and if you happen to have version 11, you can put clips side by side, or even insert a clip right over another. Previous version won't do that.
Again, youtube is your friend.