If you just want to know about port forwarding generally, what it is & how it works...there's a link here (& more links from the home page).
PortForward.com - Free Help Setting up Your Router or Firewall

If you have a modem/router that is firewalled & need specific advice for that device...have a look at what devices are plugged into your computer. What boxes plug into your computer & allow you to connect to the internet??? Look for brand name & model number. If you can't see anything, turn the device over & have a look underneath