Creating audio files can be fun. Trying to export them so that others can listen to them... not so much. This tutorial ought to help, though.
Before you do anything, first you need to download this: https://dl.dropboxusercontent.com/u/1343826/lame_enc.dll
Then, in the Audacity page of your audio file, click on the File menu button, then click on Export as MP3. It will ask you to save your new audio file, then after that, It will ask you for a Lame library. Open the area where you saved the program from above, and then It will start exporting. (This may take several minutes for it to complete exporting.)
Then, in the Audacity page of your audio file, click on the File menu button, then click on Export as MP3. It will ask you to save your new audio file.
Then, the program will ask you to find the file I asked you to download earlier.
Click browse and find your file. (Says lame_enc)
And then It will start exporting. (This may take several minutes for it to complete)