How to forward port 80 to run a server So I turned on Web Sharing on my mac and put a basic HTML file in the folder for it, but I want to know how to make it accessible from any computer. How would I do that?
It already is on your local network.  You need to know either the IP address of your Mac or name of your Mac.  You can get the name of your Mac by clicking the Sharing option in System Preferences.  It'll be at the top.  You can simplify the name to make it easier for others.  For example, if you name your Mac AppleServer then everyone on the network should be able to find it at AppleServer.local.

If you want it accessible from anywhere in the world it is much more involved.  Start by figuring out what the IP address is of your Mac and on your broadband router either set your Mac has the machine in the DMZ or setup a port forward for port 80 to the IP of your Mac.  Your router's manual should describe how to do this.

In case your Internet Access Provider gives you a dynamic IP you may also want to check out services like DynDNS so your external users don't have to ask you for your current IP address every time.