Create a main menu item, just like you were going to make that goes horizontal across the top. BUT!
Before you publish it, make the 'parent item' of the new menu item that you are creating, another menu button that you want it to 'pull down' under.
The Parent Item is what you're looking for. The 'parent item' determines which main menu button you want to be the one at the top of the pull down menu.
If the main menu item, at the top, does not need to be linked to a component or some content, you can use the 'placeholder' and type in a name of your button instead of '
'.
Hope this helps.