Follow the instructions with the Lame MP3 encoder and you will be fine.
Try to keep the size of your new ringtone to less than 1Mb so as not the use up too much BlackBerry memory. I encode them at 96kbps which is low quality for audio (in fact it's crap) but it is quite adequate for a ringtone. I also only allow the 1st 30 secs or so of the tune to save space.