Creating a sub menu item

I’ve been tasked with taking over the company website. I’m the Systems administrator and have a background in Networking. When it comes to wordpress or web site development I’m a beginner.
One question I have, is there a way I can create a sub menu item? For example our web site has a menu across the top of the page: “Home”, “About Us”, Our “Capabilities”, “Tools and Technology”, “Why Us”. My boss wants me to combine “About Us” with “Why Us” and “Contact Us”. So I need “About Us” to reveal a sub menu when you click or hover that reveals the sub menu items “”Why Us” and Contact Us”.
Any help would be appreciated.