Okay so open garageband, YOu should get this screen.
If you get a project window, just close it and it will come up.
Click create new music project.This screen will pop up
Name it, put it in a folder and click create.
When the project window opens, just drag the .m4a file into the project window.
Then find the places you want to chop, move the cursor over it and press

commandT
And export the file!
-Omi