Sound recorder for Mac OS X
If you just want a simple way to record audio, you can use the QuickTime Player application. Just choose File > New Audio Recording, and hit record.

As was mentioned already, the quality of your recording is going to be highly dependent on your microphone. The internal one is likely to pick up a lot of extra noise -- but you never know until you try. If you do get an external microphone, QuickTime Player also lets you record from other input sources (use the pulldown menu on the right side).