Here Mudassar Ahmed Khan has explained how to register and get Google reCAPTCHA Site Key i.e. Public Key and Secret Key i.e. Private Key.
In order to integrate the Google reCAPTCHA to the website, one has to register the website with Google and then generate the required Site Key i.e. Public Key and Secret Key i.e. Private Key for the reCAPTCHA.
This article is valid for reCAPTCHA version 2 and reCAPTCHA version 3.