Nick

Audacity is a decent program for editing, and has a fantastic price (free), but I wouldn't use it to save in mp3 format.

The reason for this is because the mp3 codec is not quite up to the same quality as professional programs. As a result you would be better off saving any work in wav format, and converting in something else.

To keep stuff free, an excellent conversion tool is simply using Itunes. It has a good codec, that will result in good mp3s.