Before you create a Main Menu, log in to WordPress administrator page. After that, browse the sidebar menu, you should see Appearance. Hover on it and click on the Menu. You will be directed to Menus page. You can create menu there. Click on create a menu, type in Menu Name and click Create Menu. After you created the menu, what you need to do next is to add the menu item. It can be Pages, Posts , Map Locations, Custom Links(can be linked to external web page) and Categories. After you added the menu item, select the Top Navigation Menu as Display Location for the Main Menu. Lastly, click Save Menu. to see the Main Menu you created, refresh the browser and you will see it in the header.