Drop down menu

Hello, Hope you can help me with the following. I am creating a webpage with the create theme. I really would like to get a drop down menu for my category menu. Think I have put it all right in the menu structure (main category followed by subcategories) but somehow it doesn't show as a drop down menu. I do have a main menu on top of my page (home, about, mail, social media etc), followed by title and image and below the title image are my categories. Many thanks for your help!