Summarize this article:

A sound map of the file will appear in Audacity's main display.  The file will be called lame_enc.dll for Windows and libmp3lame.so for Macintosh.  You will only be asked to do this the first time you use the Export as MP3 option. If you are converting a .wav file to .mp3 specifically to be able to play the file in iTunes then the iTunes music folder is an ideal location for the converted file.

Summary:
Download the appropriate LAME MP3 encoder. Unzip the archived LAME folder and remember its location. Download and open the free cross-platform Audacity. Select the File and choose the Open option. Locate the desired WAV file on your computer's hard drive. Select the File tab and choose the Export as MP3 option. Respond to the prompt by finding the MP3 encoder where you just chose to unzip it. Select the location you would like your converted MP3 file to appear in and rename the file if necessary.