Ok, here's a quick walkthrough, but since I don't know your router make, you're going to have to do some searching to do parts of this.
First, you need to run IPconfig. On Windows XP, do this by selecting Run from the Start menu and type cmd. You'll get a little black console box. Type "ipconfig" (without quotes) to get information about your comp. Write down both your IP address and default gateway (subnet mask shouldn't matter for this).
Now, open up a web browser, IE, Firefox, whatever's your poison, and type the ip of your default gateway in. This should connect you to your router. It's probably password protected, so check documentation to find the default password (usually there's no username and a pass of "admin" (again, without quotes)).
Once you get in, you need to find the port forwarding section, virtual server, or whatever your router calls this. It's basically a page with a big table with some or all of the following categories: Application name, port range, port type (TCP/UDP), IP address. Pick one of these and name it Sins (name doesn't really matter, just serves to remind you what that port is for). For the port, make the range 6112 to 6112. Check TCP. Put your IP address (the one you got from ipconfig) in the IP address box, then check enable.
Next you need to trigger the port. Usually the port triggering is accessed from the same page you did the port forwarding at. This has three main fields: name, trigger port range, and incoming port range. Use a name of Sins again. Trigger port range should be 0 to 0. Incoming port range should be 6112 to 6112.
That should get you ready to host.