In order to create a Server Key and .JSON File you will need a firebase console account ...
Here is the link to get your Firebase Account: https://console.firebase.google.com/
Here is the video on how to create the server key https://www.youtube.com/watch?v=iG_F3ju-dJU
You will need a package name and that is found here: https://www.youtube.com/watch?v=upwh03YkWwQ&feature=youtu.be