An extremely newbie-ish question, how to tell wordpress a file exists ?

Hello,

I’m afraid that will be a VERY newbie-ish question, themes are the thing I’m least used to customize with wordpress, thank you very much for your patience ??

In short, I don’t know how to make my blog accept (site_URL)/custompage/ to load what is actually within (site_URL)/wp-content/template_name/custompage.php
I guess there must be an easy way, but I failed to google it :-/

I created a new php file to use within one of my blogs, that page contains custom php code to allow particular stuff. I didn’t create it as a “Page” as in “wp-admin/post-new.php?post_type=page”, because it’s a mix of forms and php, it wouldn’t work within a blog post/page.

I tried to keep the fundamental required wordpress beginning and ending bits

<?php get_header(); ?>
<div id="content">
__my custom php code__
</div>
<?php include ('sidebar.php'); ?>
<?php get_footer(); ?>

Then I saved it as custompage.php … and now, I’m lost, I don’t manage to find how that could become a regular file to be shown as part of the blog.

Please, could you enlighten me ? Thank you VERY MUCH if you can ??