Before you begin assembling your photomontage, you'll need digital copies of all the pictures you plan to use. If the pictures come from a digital camera, or if you already have them scanned and saved on your computer, you're all set.
If you're dealing with standard photo prints, you can digitize them at home with a scanner. If you don't have a scanner, or if you have a lot of pictures, any local photography store should be able to digitize them for a reasonable price.
Once you have digital copies of your pictures, save them in iPhoto. Now you can open iMovie and get started on your photomontage.