Push notification links

Hello everybody,
i’m really lost, i’m working on an Ionic App and i wanna implement Notification , the server part is based on a spring restfull project, the whole idea is when a user send a payment for an other so he is consuming a service and the other user of the app should recieve this payment as a notification, i saw many tutorials but i didn’t really find what i’m looking for, would someone give me some good and important links that could help me and thanks a lot !