You can download the Firebase config file or Firebase config object for each of your project's apps from the Project settings page. You need this config file or object to configure your app to use Firebase.
You may need to access your config file or object after the initial registering of the app with Firebase if you're setting up a new environment or you're not the member who originally registered the app.
Go to https://firebase.google.com/ and go to the console.
Click on “Add a New Project”.
Enter your Project Name.
Select or create a google analytic account and click on Create Project.
Click on Android Icon.
Enter your package name (Make sure your package name is the same as your bundle id) and click on the Register app.
Once registered you’ll get an option to download your google-service.json file.