First, create a container box, to put all of your content into (all the other divs.... nav, sidebar, main, footer etc):
Then create the css:
#container {
width: 976px;
margin:0px auto;
text-align:left;
padding:15px;
border:1px dashed #333;
background-color:#eee;
}
the main thing is that after you've given the container a width, then the "margin:0px auto;" and "text-align:left;" bits are what makes the site auto-center. the padding, border, color are not needed to auto-center, but help you see where the container box is.