With most modems I have seen, when you are using the USB port for connection to a computer, the ethernet port becomes disabled automatically. If that is what you have at the moment, you will need to pop an ethernet card into your computer, pick up a decent 10/100 WiFi Router to complete your network. The reason I advise the router over a switch and a wifi access point is it is cheaper to buy the wifi router with built in switch, then either configure the modem as a bridge and use the router to control DHCP etc. with your network, or simply disable the router's dhcp and use it as a glorified switch. Also check with your ISP. I have a friend here with Bellsouth DSL and they managed to get an all in one DSL modem/router/wifi unit. It was only a b/g wifi but it got really decent speeds and it was free.
Hope this helps,