I make ringtones with an application called SoundForge - just load up an mp3 and trim to exactly how long you want and which part/parts of the mp3. Plus, you can run filters and do other adjustments on the file... ie. if its too quiet to make a good ringtone, make it louder, etc..
However, if you dont have any audio editing app like that - I know there are websites that will create free ringtones for you (well, they say free I dont know if they are scams or not, but they come from reputable BB sites). I dont know how much you can tweak their creation of a ringtone, unfortunately...
Chris