There are lots of good tutorials on how to do this. Here is just one
Photoshop How-To: Quick-and-Easy Collages
http://www.creativepro.com/story/feature/17023.html
Be sure to show us your results. Creativepro usually has pretty good tutorials.
You might search google for:
photoshop collages
or
photoshop quick mask
Also, search the archives at pbase. I am pretty sure this topic has come up before.
Good lucky, Rene