simple css/html dropdown list

I have a Italian based language site: http://www.appuntimania.com with informations about different topics.

I want to implement a simple css/html dropdown list (mobile and desktop) for my site on pages like this on the right side of the site
http://www.appuntimania.com/universita/gestione/index4.php
But the dropdown shold be SMALL on MOBILE and bigger on desktop or tablets

I want to add a new dropdown box to select category before search button is hitted.

How can i do that?

I need some html/css code suggestions, to make this.

Thanks all