This is because your modem configuration is web based and listens on port 80 by default... and it also has external access disabled... So think about this logically...
Your modem/router is using 2 IP addresses... internal/external... internal is probbaly something like 192.168.1.1 and the external can be any number..
You need to go into your router configuration and set it to listen on a different port.. (port 81 perhaps?) and then set port forwarding for port 80 to the internal (static) ip address of the box which will be your server. Let me try to draw this out...
[Modem/Router] (most simple home routers also have a built in 4-port switch which is what you connect standard computers to.)
I don't know how well that works... lol.. but connect your server just as your regular desktop computer, and give it a static IP, something like 192.168.0.10 or whatever you wish.. so long as it complies with the subnet...
Now do an install of whatever OS you wish to use for the server... If you want to be serious about hosting a real website, I recommend $70 for Red Hat Professioanl Workstation (which is ACTUALLY RHEL 3 WS) which includes all Red Hat updates and Apache 2.0 and PHP and MySQL, and everything.. or if you just want to learn how to set it up.. use whatever you want... Fedora, SuSE, Debian, Slackware... the choice is yours... If you do a Red Hat install and select all the stuff from the installation process (Apache, PHP, MySQL) it will install it all together for you, so you wont have to add the PHP modules to your httpd.conf file.
after you get it installed, go into you main www directory... generally /var/www and create a file something like "phptest.php" and include the following:
and then view that folder through apache... "http://localhost/phptest.php" and you should get a huge listing of how PHP is configured for your system. Look for a section on "MySQL" if it's there, than you have Apache, PHP, and MySQL all installed and working together... the next step is to set up your site... and get yourself a snazzy message board like this...or for a free one.. go to phpbb.com