I use zedge mobile a lot - you can download practically ANY ringtone you could possibly desire - right to your phone. Their mobile site is very quick, too, which is nice.
I just use the software that came with my Blackberry - Roxio Media Manager's quick sound editor. I edit songs I already have in my collection to begin and end in exactly the spots I want for a ringtone.
There are instructions here on Crackberry in the Blackberry 101 Lecture Series. Here's the link: