1 - Give the page a full and valid !doctype
The !doctype code is the very first line in code view, and resembles:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
(all on one line)
2 - Wrap all the page content in a container with a fixed width.
3 - Use CSS to set left and right margins of this container to auto.