Adding Pages to the menu
To add a page to the main navigation, when you are editing the page, click on the Menu settings. Add the Menu link title, most likely this will be the same as the page title. For Parent item, select <Primary Links>, that option is located towards the bottom of the list. The easiest way to control the order of the menu items is from the Menus Administration. See the Navigation Order section below.
Adding Subpages
To make a page a subpage, News > Newsletters for example, select the Page as the Parent item. Newsletters has a Parent item of News. The pages in the menu will be listed under the <Primary Links> option.
Navigation Order
The best way to control the order of the navigation items is using the Menus Administration.
Go to: Administer › Site building › Menus › Primary Links
And from there you can drag and drop to rearrange the navigation items.