How do I create an "About us" page and make it appear in the main menu?
I think there are some confusions here.

First: Custom.tpl.php-files are not custom pages.  They are custom templates for pages, and unless you need to have a different style or layout on your "about" page, there is no need for it.

Second: The hook_menu function has nothing to do with this, it is (mostly) for setting up callback functions for forms and hooking those forms up to the menu router system.

To create an "about" page for your site and make it appear in your site's main menu is much simpler.  Do as follows.


First create your "About" page as a "Basic page" on your site.  To do so, navigate to Home » Add content » Create Basic page and just create the page.  When done, press "Save".
Make a note of the node path of this page.  Let's say it is node/69.
Navigate to Administration » Structure » Menus. To the right of "Main menu", click "add link".
In the field "Menu link title", fill in the desired title (e.g. "About us").
In the field "Path", fill in the node path (e.g.  node/69 if that was the path you made of note of in step 2).
Click "Save".
Clear all caches (you always do this after changing the appearance of a Drupal site).


After going through the steps above, a tab linking to the "About us" page shall appear in the main menu.

If you want a custom layout for this page, you can create this as a template file.  If the node path to the page is node/69, the file should be named page--node--69.tpl.php.