How to achieve certain page (url) accessible to certain users

You could create a simple page template with just five lines of code:

/* Template Name: Protected Page */
if ( ! is_user_logged_in() )
    auth_redirect();
else
    require_once './page.php';

Then select that template for the page you want to protect.

auth_redirect() will send the user to the login page and back to the original URL after they are logged in.