Although both iMovie (for MAC) and MovieMaker (for PCs) are quite easy to learn, there is a bit of a learning curve if you have never edited video before. The two tutorials below do a good job of explaining how each application works.
MovieMaker Basics
MovieMaker more advanced
MovieMaker Vista Advanced
iMovie Basic
This series of tutorials from the University of Texas is better than anything on Youtube for an introduction to iMovie 4 (which is the version on the two iMacs in the TLC). Click on the link below to go to the UofT site.
You may have iMovie HD on your Mac. Click below for tutorials for iMovie HD ( which is somewhat different from iMovie 4 through 6). Some (including me) think iMovie 4-6 is actually better than iMovie HD. If you have iMovie HD 8, you can download a free version of iMovie 6 from Apple here if you want to compare the two applications.
