I would think you need a router the has WiFi capability , then you would plug the modem into the router and connect to the router with your cell phone. Some motherboards have WiFi built in. You could also get an adaptor and plug it into your pc;
What type of modem is it? It is like a home modem, or a cellular modem like a 3G modem.
IF it is just a normal home modem, I would see if you ISP will give you a better one with an eathernet port. A USB only modem is terrible for connecting up multiple devices.
IF it is a cellular or 3G modem, I would look at a wireless router that will support it. Such as something like this
Make sure you look at the compatibility list, for the router to make sure that it will support it.
Another option is to get a computer, plug the modem in to it, then share the connection to the computer's Ethernet card, then connect a wireless router in to that, via the router's WAN Port. Then you can connect wireless devices to the router.
I do this when I am stuck in a hotel with no internet, just get a little netbook and set it up. Does it great job but I wouldn't recommend this for long term solution.