themeing help for dummies

Hi,

I am trying to copy a page that has been themed. I have looked at the code and can see that a file xxx.tpl.php has been created for this. I have gone and duplicated this and made my changes and given it an original name.tpl.php. I have also seen in the style.css file that the xxx.tpl.php file is somehow being called.

How do i get drupal to call the new.tpl.php page I have created for my new page ?

Thanks in advance