You don't really say it in your post but it sounds like you can access the server OK from the other PC on the LAN but nobody can see it from 'outside'. You also don't specify what you're using for a router. Are you using Internet Connection Sharing (ICS) on one of the PCs or do you have a seperate bit of hardware for that function?
The key phrase you're looking for is 'port forwarding'. Web requests typically are addressed to port 80 though it can differ in some circumstances. You have to configure your router to forward any requests addressed to port 80 to the IP address of your server.
Another consideration is domain name. Is your WAN IP address registered with Domain Name Servers? If not, then anyone on the outside will have to know explicitly what your WAN IP address is to send the request (that is, by typing your IP address into the address bar in their browser).
That's about all I can do with the information provided. Hope it helps.