How do I make it so that only a logged in user can view certain pages?

Hi,
First of all, let me say that I’m a real newbie when it comes to doing more complicated stuff with PHP, and the like. Following this tutorial: http://www.devarticles.com/c/a/PHP/Creating-a-Membership-System/

How do I make it to where certain pages cannot be viewed unless the person is logged in?
Thanks!