Hello,

To create a menu and add items to it (drop-down menu), just follow the basic wordpress rules for creating a menu.
Go to Appearances > Menus > Add New. Set the menu name, save and continue adding more items under it. (read this article for more details).
To customize it using theme’s options, go to General Options > Header Options > Dropdown Options & Mobile Menu.

Let us know if you have further questions.

Best regards!