Instructions on how to do port forwarding are hard to give since each brand of router has a different way of doing this. As outlined above many routers allow you to access a configuration screen by using your browser to go to the gateway, which is located at http://192.168.1.1/
for my router.
My router is a Linksys and there is a tab for port forwarding on the configuration screen. From there you put in what port you want to forward and to what IP address you want the port forwarded, usually 192.168.1.100 if only one computer is hooked to the router.