As you create new pages in a Google site, they will appear automatically in the menu of the site, with each page showing as a main menu. You can change the hierarchy of your pages to create submenus in the site, but can currently only create one level of submenus.
The following screenshot shows 5 pages on the same navigation level:
In the following screenshot, two of the pages have been moved to submenus under another page:
To set the hierarchy of pages:
Click on the Pages tab on the pane on the right-hand side of the screen
Click on the page you wish to make a submenu of another page
Drag the page over the page you wish it to show under in the menu
Release the mouse button to set the page navigation