Joan,

Audacity does not create .mp3 without what is called a 'codec'. At some point in time, you have to tell Audacity what codec to use and how to use it. The 'how' would set the bit rate.

The .mp3 files from Audacity have a bit rate, probably 128 kbps, and probably generated using the LAME codec.

If you are just 'putting music out there' for friends and family to hear, and not necessarily download, 128 kbps is fine.