Here's how you do it:
1) Install Apache on your computer.
2) Install MySQL and Perl support onto Apache (I think it comes with it)
3) Install MySQL onto your computer in order to make a database.
4) Visit the loopback address, localhost (http://127.0.0.1)
IF YOU WANT YOUR WEBSITE TO BE ACCESSIBLE FROM THE OUTSIDE:
4) Open a port on your router to your computer (I think it's usually port 80).
5) Find out your external IP address at http://www.whatismyip.com/
6) Enter your external IP address into your web browser. If you get a page, your computer is now a web server.