I don't know much about this, but I know how to with iTunes... All you have to do is change example.m4a to example.m4r, and drag into iTunes.. But that only works with iPhones.
Otherwise, use http://www.iwfr.net/mp3toringtone/ (it's what I use)
well first off mp3's arent for ringtones but they can become it .a ringtone file type is usually an .m4r, m4v or an mp4 so you would need a converter for it