Once you have finished a project in GarageBand you may want to export the project as an MP3 file. To do that, take the following steps:

1. In the GarageBand menu, click Share and then select Export Song to Disk.

2. In the following window, select MP3 as the file type, name the file, and select a location to export the file.

3. Click Export to finish the process. 
4. You can then find the MP3 file by navigating to the location on your machine where you chose to save the file in step 2.