How do I make the mp3 audio excerpts on my computer?

iTunes, from Apple, is freely available on Macs and PCs. Here's how to create an mp3 file in iTunes. There are various other ways to create audio excerpts using free or low-cost programs. Here are links to a few. (The Copland Fund does not endorse any of these programs and is providing the links only for ease of reference.)
Snapper
Audacity
Quicktime
Windows Media Player
Sound Forge
Goldwave