Creating a menu
To set up your menu, navigate to Appearance > Menus. Here, you can edit a menu or create a new menu. When creating a new menu, give it a name, such as Main Menu and then click Create Menu. You can now start adding menu items, such as pages, posts, custom links and categories.
To change the order of the items in the menu, simply drag and drop them into position. To create drop down menus, for subcategories of a category for example, drag the menu item to the right so that it sits below the parent menu item.
Please note, you do not need to create a new page for a category, a category archive is automatically created, you just need to add the category to the menu.
Once you are happy with the order of your menu, scroll down to Menu Settings and tick the box for Main Menu under Theme Locations and then save the menu.