First, create all pages you need to build a structure with.
than, go to the page you want to make subpage and apply a parent page to it. You can do it in the page settings.
than, the page is seen in the tree view as a subpage.
in order to display it on the native main menu, you need first,to override the main menu module to make it recognize our structure.
actually, that's all. Than you can go to the main menu and see the AdCMS block in the left part of available blocks.
so you can handle it like any other block in this menu and apply a parent page to your active pages. The submenu will be created than.