You can have a look at this web site.
Drop Down Menus[
^]
Especially this one looks like what you want it to look like:
Responsive Flat Menu[
^]
You should also consider to look into
Bootstrap[
^]
Quote:
Bootstrap is the most popular HTML, CSS, and JavaScript framework for developing responsive, mobile-first web sites.
Bootstrap is completely free to download and use!
In Bootstrap, you have this kind of menus already built in.
Lycka till.