Create a menu
Go to Appearance > Menus and create a new menu. The name of the menu is not important. Add the pages to the menu and save the menu. Set the menu to Primary Navigation Menu under Menu Settings. Save the menu.
Subpages
The navigation is designed for using subpages. The subpages will show in a dropdown menu. You can create submenus by placing a menu item under the parent and indent it to the right.