Came across this FREE program and it rocks. It allows you to cut portions of your favorite MP3 to use as a ringtone. Instructions are super easy. It's called MP3 Cutter.
Each cell is different how it stores music files so you have to do some legwork there. If you are on Virgin Mobile like me, I know exactly how to save MP3's for ringtones.
Look for another program called Format Factory. It re-saves your MP3 into another MP3. Don't know why but you gotta do this with your ringtone. Rename the MP3 to a .3g2 file. PC will ask if it's okay just say YES. Drag this onto your cell.
Once it's on your cell, go and find the file, then select Assign and Assign it as your new ringtone. Good luck.