How do you convert cda files to mp3?

You can't, however, you can copy a CD or DVD. There are lost of free programs, try Windows Media Playet, latest version is a free download, or try FreeRip, Ashampoo's good but you have to buy it.

Basically, it's dead easy.

You can save the CD/DVD Files to your hard drive & play them and/or copy/burn them to blank CDS & DVDs