To avoid having too many top level menu headings it is always good to group things together. This means you can have multiple layers of menus but always remember that your users want to navigate easily around your site. Keep it simple!
To place the Page (menu item) under a top level menu item select the Parent from the drop down in the Page Attributes box when creating or editing the Page.
Each sub menu can be ordered by placing the number of its position in the Order box in the Page Attributes box.