Firstly read the full documentation of how to add push notification in any android app How to add push notification in android application from android studio – Android developer (part – 1 Connect with firebase ) ~ http://androidrace.com/2016/12/08/how-to-add-push-notification-in-android-application-from-android-studio-android-developer-part-1-connect-with-firebase/
How to add push notification in android application from android studio – Android developer (part – 2 Working with server) ~http://androidrace.com/2017/01/05/how-to-add-push-notification-in-android-application-from-android-studio-android-developer-part-2-working-with-server/
If you face any problem dont forget to share :)