What software should I need to convert a RAR file to mp3?

First you need to open the RAR file since it's not an audio file, it's a compressed archive (like a ZIP file).
WinRAR - Free software downloads and reviews - CNET Download.com (http://www.download.com/WinRAR/3000-2250_4-10007677.html)

Then extract the audio files contained within and post to use here what format they are in, we can suggest a converter.