Your talking about Personal Web Sharing.
It opens port 80 and allows you by default to place web pages in two main locations. One is the system default area at /Library/WebServer/Documents/. You can place a new index.html file there to override what is coming up now.
The other location is ~/Sites/. The tilde represents the short username. If you short username is joe than the full Finder path is /Users/joe/. In the browser type
http://localhost/~joe/ and it will find the default index.html file in joe's Sites folder. From the outside, a friend would type
http://nn.nnn.nnn.nnn/~joe where the n's are your routers external IP address.
You'll need to learn how to forward port 80 from you router to the Mac. On the router I have the option is titled Virtual Hosting. You enter a port number and the destination IP address. You'll likely have to create a fixed IP address on your Mac because if your using DHCP, the address can change under your feet.
Note that some ISPs block port 80 so people out on the net couldn't see you page. If that is true for you, than you need to change the port in the router and in the Apache (webserver) configuration file. That file is /etc/httpd/httpd.conf which can be found via the Finder by Selecting the "Go to Folder..." selection under the Go menu item. I'm looking via Tiger right now. The usual alternate port to user is 8080, but you could use almost anything above that if your ISP also blocks that one.
Enjoy. That is the easy part!