Follow below Steps to create JSON file :
Step 1: Go to Google Developers Console. If you have a Google account, login, else Create an Account.Step 2: On the dashboard, click on the Project dropdown menu.Step 3: Select an existing project or create a New Project.
Step 4: Click on Credentials.Step 5: On the credentials page, click on Create Credentials.Step 6 : Select Service accountStep 7: Enter the service account name, service account ID, and description, then click on Create.Step 8: Select a role from the dropdown menu and click on Continue.Step 9 : Fill in the required fields to grant access to the services account, if required. Click Done to finish creating the service account.Step 10 : Under Service accounts, click on the Edit icon.Step 11: Click on Add Keys dropdown menu.Step 12 : Click on create new key Step 13 : Select key type JSON and click on Create.Step 14 : Your JSON file will be created and saved to your computer. Once done, click on Close.