AijoNeko, if you can't implement mod_rewrite like moshu suggests (which if you can, is easiest), then here's the steps for setting up an external php page to use WordPress theming (and to display your Page content):
1. First, add this to the top of your php file:
Make sure the path to wp-blog-header.php is correct.
2. Set up the layout in your file to match that of the templates in your theme. Quickest way to to do this is to copy all the layout and php elements from the theme's index.php into your file (if the theme has a page.php, use this).
3. Install my Get-a-Post plugin, and use the
get-a-post() tag in place of The Loop in your custom template to call the specific Page to display.