In the past I've ranted about the lame quality of most built-in camcorder microphones and the importance of audio in a video production. Assuming your camera will accept an external mic, (and I hope it does) here's a link to Audio Technica's, "A Brief Guide to Microphones". The company provides an excellent basic primer on how and why microphones work, and which mic is best for the job at hand.
I'll also add that you should purchase the best microphone you can afford. Speaking from experience, you can get burned going cheap on a microphone purchase that in the end doesn't work well.
In my experience Audio Technica's equipment is affordable and of very good quality.
And while cameras and recording formats seem come with unsettling regularity, a good mic will last for many years.