You will need Client ID and Client Secret Key to get started.
-Client ID & Client Secret Key : Follow the step by step tutorial mentioned here to get your Client ID and Client Secret Key.
Add the Client ID & Client Secret Key & hit Save button.
After saving Client ID & Secret click on Authorize Button.
Signin using your google account & allow access to sheet & drive api.
Go to Ninja Forms and edit one of your Forms.
Click on Email & Actions & then select Google Spreadsheet.
Now Select your spreadsheet & map sheet headers with Form Fields.
NOTE: First row in your sheet are the sheet headers.
That’s it .. Now when you submit the form it will add new row in Google Spreadsheet.
You can download the plugin from here