Did you register your domain yet?
Register your domain here - https://www.google.com/recaptcha/intro/
Than add the below code to your core/local.config.php. Replace PUT KEY HERE with your keys.
/* Keys for recaptcha, you can change these if you want to your own but it's a global key so it should just work */
Config::Set('RECAPTCHA_PUBLIC_KEY', 'PUT KEY HERE');
Config::Set('RECAPTCHA_PRIVATE_KEY', 'PUT KEY HERE');