First off you need to find a song that have you. I'm sure you can use DRM-free
(DRM is locked music like you buy from iTunes and CD's....You need to have the file unprotected)
(can can always get free music off of myspace or download.com)
What you do is you drag the music file to your desktop for easy use.
Then download a free audio editor called http://audacity.sourceforge.net/