Have a look on YouTube at other book trailers to get an idea of what yours could be like. Aim for no longer than two minutes in total. Here's mine http://www.youtube.com/watch?v=BpNB65nYyRM
1. Familiarise yourself with the Movie Maker program by using/importing images in from your own files, adding bits of music you might have in files.
2.Choose your photos. Think about the type of photos you could use. This will probably be the most time consuming part. You can get free royalty free pictures online. I got mine from here: http://www.dreamstime.com/free-photos You'll need to download them and save them into a folder.
3. Choose your music. Remember to think about the age and genre you are writing and how the music will help to get across your message. I found free royalty free music here: http://teknoaxe.com/cgi-bin/home.pl Obviously, you can choose from a different site, just be sure it is free to use and royalty free. Download your music and save to your folder.
4. Now for the actual making. I googled 'How to make your own book trailer' and this site came up.
It was really useful and gives lots of other ideas as to what to do.
Take your time and enjoy the process. Sit back and watch your book trailer!