the easiest way to do this is to just use a dynamic language, (for this example I'll use PHP) and have a wel organised folder structure, again I'll make some suggestions...
First the folder structure, I suggest that you have a root folder (obviously) and in this page you put a file called index.php and a file called menu.html and sidebar.html
inside of the root folder you have a folder caled pages, inside of this folder you put all your content pages. eg, home.html, links.html, gallery.html
now here is the part that achieves what you want to do...
Make the index.php page like this.
(you might want to change the layout etc...)
$page = $_GET['page'];
//first start the html layout of the page.
now you see that when you browse to the site you automaticall see the page home.html, you make your menu links in the follwing way...
<a href="?page=gallery">View Gallery</a>
Hope that was helpfull, if you don't understand anything or need an example in a differnt languae (web language -I only seak english!) then just ask!