I've done some movie editing with iMovie on my Mac and it works well enough.
Unless you want to invest in something else, iMovie is your best bet. It's easy enough to use and it's actually not that bad.
If you want to invest in an app, then there are a few products you can look into...
Adobe Premiere and Final Cut Pro (Apple) are popular choices. They're a lot better, but also more expensive.
However, my pick for a commercial app to work with would be Avid Xpress DV. Of course I'm a little biased since I work for Avid (a company that they own, more precisely) but even if I didn't it would be my pick for a commercial application to do video editing with. Seriously!
Hope this helps.
If you start working with iMovie, let's say, and you need a hand figuring things out, let me know. I'd be happy to help.
