When I began my quest to learn how to podcast with my students, I viewed the Apple tutorial located at http://www.apple.com/ilife/tutorials/#garageband-podcast. Apple seems to have routed that address to iLife '11 as of 12/13/10. However, I have also found an excellent professional development site put up by Henrico County VA schools at http://teachers.henrico.k12.va.us/staffdev/maddux_j/podcasting/Home.html. There is also a blog entry, especially helpful if using a slightly older version of GarageBand, at http://www.voiceovertimes.com/2008/01/04/10-tips-for-editing-a-podcast-in-garageband/. By the time you watch all of these, you will be an expert!
Here is a sample of the podcasts my students created in class. You can sense their excitement. All of my students, from very different backgrounds and ability levels, were able to participate in this activity. Demo podcast
I hope you enjoy it....and try this with your students!