1- Creating and activating the menu
You go to the dashboard, navigate to Appearance –> Menus –> type the name on Menu Name box and click Create Menu (you can ignore this step if you have already a menu)
Then, check the box “Primary Menu” to assign the current menu as the menu bar at the Top.
2- Adding the items
Just simple navigate to the tabs such as Pages, Posts, Categories… to add the items to the menu
To create a dropdown menu for categories, you add the items from Categories tab to the menu, then drag and put them like this:
Note: To create the categories, you navigate to Posts –> Categories.
Be sure you save changes when you finished.