Download a free audio editing program that does mp3's. Just from a quick google search it looks like there are a lot of them.
This looks like it does what you need.
Use the program to open up the mp3 file that you want to turn into a ringtone. Cut it down to the 20 seconds or whatever that you want as a ringtone and Save As.
Transfer to phone.
Set as ringtone on phone.
I started making ringtones when I got a phone that didn't have a strong vibrate and haven't ever purchased one since. I tend to like instrumental ones with no lyrics and currently have "Sigur Ros - Gong" starting at the 0:34 mark as my default and it's probably the sweetest ringtone I've ever heard. [img]/images/graemlins/smile.gif[/img]