You need an MP3 encoder to do this. From what I've heard, the most respected free (though using it supposedly violates some patents) MP3 encoder is LAME. LAME itself isn't very easy to use if you're not familiar with commandline tools, but the LAME page contains links to GUI tools that use LAME as the encoding engine. More information on LAME usage/settings and MP3 encoding in general can be found at this site.
If you're willing to pay for an MP3 encoder, the most respected one is Fraunhofer's l3enc/MP3enc. LAME is supposed to be pretty competetive with it, though.