To use reCAPTCHA verification, you need to generate api key, To generate key click here. There you need to generate key and fill form

Now install module and go to Settings >> General Settings. In that you can see Google reCAPTCHA Validation section. Click check box and add API site key and Site secret key which you have generated from google admin console.

reCAPTCHA verification in login page

reCAPTCHA verification in signup page

Alert when wrong verification or without captcha verification