What type of system do you have? Laptop or desktop? Does it already have WiFi hardware?
Is the modem built into the machine, plugged into a card slot, or a separate device that is currently attached to your machine with a cable, or through your network?
My problem is that I don't know if you are trying to attach two separate devices with WiFi, or make a machine share its modem connection with other machines via WiFi. I know that you will need WiFi hardware for at least your PC. But I need more details to give a meaningful answer.
Answer without clarification, since people seem to demand these: Buy a wireless access point and connect it to the modem's network port. Buy a WiFi ExpressCard and plug it into your notebook's ExpressCard port. Set them up to talk to each other.