As I wrote over in the other copy of this thread, "Buy a WiFi ExpressCard and plug it into your notebook's ExpressCard port. Set them up to talk to each other."
If your machine is not a notebook but is a tower, while you can buy a WiFi card it's probably simpler to buy a USB Wifi "dongle" and just plug it into an available port. Here's an example; I am not endorsing this product: http://www.newegg.com/Product/Product.aspx?Item=N82E16833166068
Please be aware that there are several different standards for WiFi. You must find out which standard the modem supports (tell us the make and model and we can look it up) and buy a compatible device for your computer. The most common versions are 802.11a, b, g, and n. http://en.wikipedia.org/wiki/802.11
For setup, it is very likely that your modem will act as the control point and you use WiFi setup on your PC to attach to it. Once you have done that, I STRONGLY suggest that you find the manual for the modem, read the section on security, and secure your network. If you leave the defaults, anyone can attach to your network and steal your soul.