Open your GarageBand project
Click Share in the menu bar
Select Export Song to Disk
Choose MP3 as the file format
Select the audio quality settings you want
Choose the destination folder
Enter a file name
Click Export
Wait for the export to finish
Open your GarageBand project
Click Share in the menu bar
Select Export Song to Disk
Choose MP3 as the file format
Select the audio quality settings you want
Choose the destination folder
Enter a file name
Click Export
Wait for the export to finish