<style type="text/css">
#main { width: 100%; margin:auto; }
</style>
....

<div id="main">
content
</div>

If you want "no margin" so the content truly goes up against the edge of the window, you will have to zero out the page margins:

<style type="text/css">
body { margin:0; padding:0; }
#main { width: 100%; }
</style>