You should get notifications by default after installing the Planning Center app on your device. If, for some reason, you aren't getting notifications, you can check your settings following these steps:
Open the side menu by swiping left-to-right or by clicking the menu icon at the top-left of the screen.
At the bottom, click the Settings button.
Now, click Settings again. [Yeah, I know.]
Look for the section call Scheduling request notifications, and make sure that is set to Enabled.
You can also enable Email notifications on the settings page.