Regarding software, there are plenty of solutions... Garageband is a very good one. If for some reason you don't like garageband (or can't use it) try Audacity which is freeware and very good.
If you've upgraded to Snow Leopard, you can use the Audio recording in free Quicktime X. Or, if you haven't upgraded (but are willing to spend some money), you can purchase Quicktime Pro, which offers that same function.
And then there are dozens of 3rd party audio recorders (most of them cost money). You can check MacUpdate or iusethis to check them out.
But more important than the software you use to record it, is the microphone you use, as it being the first link in the recording chain.