How to add second level drop down to main menu item?

I am a novice when it comes to website customization and was wondering how I could add a drop down menu to one of my main menu items? I’m wanting it to where I can hover over the word “Nutrition” on the main menu and it will drop down with the words “Diet” and “Supplements”, each of those words leading to different pages. I tried doing this by going to Appearance, Menus, and creating a new menu, then adding the sub menus below and slightly to the right of the menu item, but it didn’t show up when I saved it. Thanks in advance!