How to hook into user registration process Before user registers

You need to create your own hook for registration_errors filter:

add_filter( 'registration_errors', 'wpse8170_registration_errors', 10, 3 );
function wpse8170_registration_errors( $errors, $sanitized_user_login, $user_email ) {
    if ( /* something happens */ ) {
        $errors->add( 'myexception_code', 'This is my message' );
    }

    return $errors;
}