by default, your library should already exist in 2 places... on your ipod, and on the hard drive of the PC where the tunes came from. So if you're asking about how to back up the files on your PC, just grab a couple of blank CDs and burn all of the MP3s in the library to CD. Or if you have access to a DVD writer, you can get more MP3s per disc that way.
Hope that helps,
Chris