$user_id = wp_insert_user( $user_data );

$this->password = wp_generate_password(6, false);
$user_data['user_pass'] = $this->password;

function prepare_email(){

        $confirmation_to = $_REQUEST['email_address'];
        $confirmation_subject = 'Confirmation - Registration to My Site';
        $confirmation_message = 'Hi '.$_REQUEST['first_name'].',<br /></br />Thank you for registering with My Site. Your account has been set up and you can log in using the following details -<br /><br />'
            .'<strong>Username:</strong> '.$_REQUEST['username']
            .'<br /><strong>Password:</strong> '.$this->password
            .'<br /><br />Once you have logged in, please ensure that you visit the Site Admin and change you password so that you don't forget it in the future.';
        $headers = 'MIME-Version: 1.0'."rn";
        $headers.= 'Content-type: text/html; charset=iso-8859-1'."rn";
        $confirmation_headers = $headers.'From: My Site <no-reply@mysite.com>'."rn";

        $this->form_for_email = compact('confirmation_to', 'confirmation_subject', 'confirmation_message', 'confirmation_headers');

    }

$user_id = wp_insert_user( $user_data );
wp_new_user_notification( $user_id, $user_data->user_pass );