Here is a guide to creating collages using Picasa. If you don't have it, or know what it is, it's a free photo organizing and editing software from Google, and I heart it. It's compatible with Windows, Mac, and Linux, and you can download it here.
You can find one of my scrapbook layouts with collages I created posted here. I hope that this helps you out, but if you have any questions, please feel free to email me!