Secure pages

Hi everybody,
i have a user login authentication sysytem on my machine here where users
may view pages but to do other things the user must login. the login
scripts seem to work ok and are based on a "Secure Authentication System"
from www.phpbuilder.com. Basically the page where i want people to arrive
at upon logging in is at present viewable by anyone who puts the proper
url into their browser. how do i keep this and other pages secure from
people who havent logged in?
Thanks,
Paul