How can I create a new page template for the user registration page?
There are so many ways to theming your registration page.
You can create user-register.tpl.php

To know about how to create user-register.tpl.php refer below links.

https://www.drupal.org/node/1211520

https://api.drupal.org/api/drupal/modules!user!user-profile.tpl.php/7

https://www.drupal.org/node/1195754

If you want to know how to create tpl files.

Page template suggestions are not working