For ready-made ringtones, larryo108 has your answer, but.....
If your talking about MP3's as in full songs, I would really suggest using a ringtone creation app like Vito's Ringtone Editor:
http://vitotechnology.com/en/product...oneeditor.html
Full MP3's will take a LOT of memory, and you only need the first 15-25 seconds anyway. Vito does cost ($7 I believe), but if you plan on using a lot of custom MP3 ringtones, it is easy to use, saves in your ringer folder for you, and does all of this right from your WX.