you would generally have to get the port that the service uses (ie for http its 80 by default) and set the router to forward all the requests to that port to the computer running the server,
if its a local server you want, dhcp should take care of that, basically type in the assigned ip address in the address bar ie 192.168.0.1 or whatever. you could also type in the hostname like: http://upstairscomputer if the computer has a hostname of "upstairscomputer" assigned to it