Menus are vital to customers navigating your website. 

It's essential you plan these in advance to ensure you build the menu trees correctly. You should always break down how you want your customers to navigate. For example if you wish to sell a Blue Male hoodie, your category structure may follow, Male Clothes -> Hoodies (as a sub menu). These will also match your category structures. 

To add in your menus, 

Head to the Content Menu 
Select Menus 

You can add a new menu or edit the exiting 

To update the main Nav bar we use the title Header for the parent menu 
Navigating inside this via the black arrow allows you to create the sub menus such as page links 

To ensure the menu items link to areas of the website or external, you simply click on manage and edit the navigation either to an interal link or add a custom external link.