Without knowing what connection you have and whether you're on a LAN at
home, it is difficult to tell you exactly what to do.
One of the problems with online gaming is finding your host IP (the one
assigned to you by your Internet Service Provider) and whether you're
running a dynamic or static IP. If you're on a dynamic setup where each
time you log on you get a different IP, you can check it at
www.whatismyipaddress.com.
For opening ports, it depends on the game itself (some have rather odd port
requirements), and you'll need to read the manual, check the support site,
or some of the gamer sites to determine which ports are used when hosting a
game server.
You need to check the configuration of your router and/or modem. Same thing
there, read the manuals and check the support sites. There is a pretty good
explanation of everything involved here:
http://www.u.arizona.edu/~trw/games/ports.htm