How do you convert a music file to a ringtone?
It depends what format of ringtone your phone supports. If it is
a newer model, simply copy the .mp3 file across to your handset.
Otherwise it is probably a .mid file or polyphonic ringtone. There
are many programs with which you can create your own polyphonic
ringtones. Otherwise simply do a quick google search for the song
you want, followed by .mid.