How to make a horizontal – drop down header menu

Hello, I’ve searched all the questions at the last 11 pages and I’ve tried these two answers:

how do i add a header menu

&

Make the menu drop down

I’ve tried to combine these but unfortunately I couldn’t make it!
I would like to have a header drop down menu with sub menus.
For example:
First Level:            List1                    – List2               – List3
Second Level:       item1                   item1
item2                        -subitem1
-subitem1              -subitem2
-subitem2         item2
-subitem1
-subitem2
etc…
Every level will be shown after the hover of the parent <li>!

Could you please help me?