I guess you want to center a fixed width 'column' in the page containing your site's content? Try this :
1. Add the following into your CSS file
#content {
position: relative;
left: 50%;
margin-left: -300px;
width: 600px}
2. Enclose all of your page content (ie all between <body> and </body>) with
<div id="content">
your page content goes here
</div>
This will set all your content in a 600 pixel wide strip that centers in the browser window. To change the strip's width, adjust width as needed and always keep left margin so it satisfies:
left-margin = -0.5 * width
Hope this is what you want and that it works for you :)
There are other ways to achieve the same thing in CSS, but I am not familiar with them (yet) ;)
