1) Get
Audacity2) Load your MP3 into it
3) If your song is stereo, split the stereo track (arrow by track on left hand side)
4) Delete one of the resulting tracks and convert the other one to mono
5) Save as .OGG in your Add-Ons folder
6) To use it in your server, add it to musiclist.cs OR before you start a server, tick it on in the music files dialog.