reCAPTCHA. Googl?e

Hi
I want to use the reCAPTCHA of Google in my forms.
Google has given me the
– Site key
– Secret key

Apart from pasting this snippet before the closing </head> tag on my HTML template:
<script src=’https://www.google.com/recaptcha/api.js’></script&gt;

what else do I have to do?
Thanks