First,open Audicity
Then open first song that you want to mix
After that click Project>Import Audio,to import the second song you wanna mix
Then click file>export as WAV(this software cant export it as MP3 format),so you need a media coverter to convert the WAV sound to MP3 sound.
To download Audacity:http://www.download.com/Audacity/3000-2170_4-10058117.html?tag=mncol&cdlPid=10606824
To download media converter:http://www.download.com/Free-Mp3-Wma-Converter/3000-2140_4-10442362.html?tag=mncol&cdlPid=10902033
