The most popular way of centering your content is to put your whole body in one div with and id (ie container) and add this in your css code:
div#container
{
margin: 0 auto;
width: (the width you want, could be in %, px, or em)
}
The margin attribute can have 2, 3, or 4 values. If it contains just 2 values (like in the example above), the first value is for the top and bottom margins and the second value is for the left and right margins.
When you specify 'auto' whatever space is not used by your width gets divided into two. So if your layout is 800px and the screen width is 1000px, the 'auto' value makes a 100px margin on the left and right side.
FOR CURIOSITY'S SAKE...
Just for reference, the html's body looks like this:
<body>
<div id="container">
... all the content stuff here...
</div>
</body>
When you use 4 values in your margin attribute, they are specified in the following order: top, right, bottom, left (clockwise from the top). I can't remember what's the 3-value for. hehe