how to create my own php custom page?

Hello,

When creating a custom page template, you need the following line of code to start off with:

<?php /*
Template Name: Custom Template
*/ ?>

Replace ‘Custom’ with whatever you want your page to show up as in the page editor (the image you linked to).

Then go about coding your custom.php file as you had intended.

Hope this helps.

where i need to put thet custompage.php file? in which folder?

Are you creating your own theme?

No.

Then create a Child Theme: https://codex.www.ads-software.com/Child_Themes

And then put your template file in your Child Theme folder