You'll need to find the model name of the router or router/modem combo you're using. This shouldn't be too hard. Usually, the internet comes in through your modem (box connecting to the wall either through phone line or cable line) and is then passed to a router which sorts and distributes your internet connection to the computers in your house. Go find the latter, turn it over, and look for a brand name and some kind of model number (Ex.: Linksys wrt54g, D-link DI-804). Once you have that go to this link and locate your model:
Follow the instructions provided. The ports you need to open are:
28902 - not in the readme, but opening this port helped alot of people
Once you're done click each port to test if it's really forwarded. Bear in mind that sometimes it takes a minute for the router settings to apply (occasionally you'll even need to reset it before the settings take effect).
Forgot to mention!!!: You'll need to either add an exception to the windows firewall or disable it altogether. I disabled mine; it's all but useless. I'll write up a guide when I'm done with class.
There you have it! Now people can join your games.