Message box which I can close (symbol "x" to desappear it)

Hi, I am developing an italian Company website. A few days ago, a new law came into effect requiring anyone visiting a website to be informed that there are some cookies (technical, statistical, etc.) that could trace its behavior while browsing the site.
I need to insert at the top of the site a sort of banner in which to place text that informs visitors on this subject. I wish, once the visitor has read the box content, to have the chance to close it, deleting it clicking for example on the “x” symbol up-right, again having the home page but now without the box.
For example, please visit http://www.alitalia.com/it_it/. At the top of the website, you have the availability to read more informations about cookies or to close the message box clicking on the rounded “x”.
How can I do to do so? I am currently using a “promo-box”, but you can not close it, that is always visible on the home page.
Can you help me? Many thanks.