To configure your recaptcha you will need a google reCAPTCHA Site Key and a google reCAPTCHA  Secret key.To obtain these API keys follow these steps:

1. You need to sign up on http://www.google.com/recaptcha/admin .
2. Click on Get reCAPTCHA button.
3. If You don’t have any sites registered, then Register a new site by your domain. [ reCAPTCHA will work for its sub domain automatically]
4. Then you will get Site Key and Secret key.
5. Go to Frontend Settings >  Site Settings >  Third Party Settings >  Captcha Settings of your admin panel and set API keys.
6. Turn on the reCaptcha status.