Go to http://www.phonezoo.com/home.do and sign up. This site is totally free. Be sure to leave the 3rd Party Offers box unchecked if you don’t want to get spammed.
On Phonezoo, go to Home > Create Ringtone. Browse to your audio file and upload it. If you didn’t get your music from a Creative Commons site or make it yourself, it’s probably copyrighted. Check the appropriate box.
Use the Phonezoo editor to choose which part of the audio file you want as your ringtone. Preview it by using the play button, and practice using fades!
When you’re finished editing, choose “Save and Send to Phone”.