back button and menu navigation

Okay, I'm trying to design a mobile version of my site and am stuck on my menu.

Basically at the moment I've got a simple onClick running on the page so if the user decides they don't want to view click whats on the menu then they can click the page and the menu disappears.

But what I really want is a way of using the back button on the browser to return from the menu. I'm guessing it takes some kind of event or setting something, but am at a loss.

I'm a newbie, so be gentle, I'm hoping the community can help me a little as to where to start.

My main question is how to use the back button on a browser within my HTML or JavaScript, any ideas as to how to proceed would be greatly appreciated.