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.
Go to Forms and edit one of your Forms. On the tab click on Settings => Google Spreadsheet Add-on Feed.
Add the new feed & select spreadsheet & sheet to map sheet headers with a 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 contact in Google Spreadsheet.
You can download the plugin from here