Software needed:

• Audacity – free, cross-platform (Mac, PC, Linux/Unix), able to produce sound files in multiple formats, including mp3 and edit sound files; 
• iTunes – free, cross-platform (Mac, PC), good for importing audio files from a CD and exporting mp3 files; available for download at http://www.apple.com/itunes/
• Other downloadable software:

  Adobe Audition (used to be Cool Edit - PC only) from http://www.adobe.com/special/products/audition/syntrillium.html

  Quick Time Pro (Mac & PC) from http://www.apple.com/quicktime/buy/

Download (save) audio files from the Web

Go to a website that has music or audio you can download, such as http://freekidsmusic.com; or open a search engine such as http://altavista.com and search for an audio file. Try to find one that is in mp3 format.

Use Wes Fryer's list of good free music locations

Larry Ferlazzo has a list of Best Places to Get Royalty Free Music and Sound Effects.  This could be a great resource when developing podcast episodes.