To enable reCAPTCHA, grab a public and private key pair from here and add them to a config file. You can also go into the moderator dashboard and go into Configuration and put these keys into the recaptcha_public and recaptcha_private boxes respectively.

$config['recaptcha'] = true;
$config['recaptcha_public'] = '<your public key>';
$config['recaptcha_private']    = '<your private Key>';