Once logged into WordPress go to;
Appearance >> Menu >>
  1. Create new menu.
  2. Give your new menu a name.
  3. Select the location you want the menu to display
  4. Search in pages for which pages you would like to add to the menu.
  5. Reorder your pages to the order you want them in.
  6. Save menu


Once you have added your new page to the menu, it will appear in the menu structure.
In the menu structure, you can rearrange the order of your menu/ nav and even create a drop-down.
Simply drag the pages into order or place under other pages to make them sub items (drop-down).