I finish my homepage layout.html page. what i want to do is just centering my web page and the whole site. but it doesn' t center.

I'm doing a site and following the directive in sitepoint book "standard compliant website using dreamweaver 8" using strict 1.0 and includes.

i want to center the homepage-layout html before i create the include files.
i want the site to be 900px wide, not 100% wide.

How do i go about this?