How to Add Menu Items
In your admin Dashboard menu, click on "Appearance" and then "Menus." In the Menus page, you will be able to select the Pages that you have created on the left sidebar. Click on "Add to Menu" to add your selected items to the menu. The menu items will appear on the bottom of the list, but you can scroll down, click and drag the item(s) to where you want it displayed.
In the Menu Structure, the items all the way to the left are the main menu items and will display in your main menu bar. When you drag an item under the main menu item and slightly to the right, it will become a sub-menu item for the main menu item. In the example above, "Affordability Calculator" is a sub-menu item of "Resources."
For more information on editing menus, check out the official WordPress Menu User Guide.