Test For Push Notifications With One Signal and Pushe Services
go to the link and after Create an account and log in . Create New Android Project. after This You Should go to Android Studio and Write Some Codes According The One Signal DOCUMENTS and DOCUMENTS and Install Your App ...
for generate a google server api key go to this or this video on youtube
After That You Can Send New Push To Your Users...
go to the link and after add New Application you can Read All Tips for Run This in Your Android App...