I use Ahead Nero, it has an audio editing feature.  Use that feature to open your favorite MP3 and cut the piece of song you want to make into your ringtone.  It should be less than 25 seconds.  Then convert the sample format to 8000 Hz(sample rate) and 8 bit(bit depth), as this is the  
specs for a .wav ringtone. Save as the name of your ringtone, and use webjal to upload it to your phone via data cable.