2 Ways.
1) Get rid of your USB modem, get a Wireless ADSL router (eg.Netgear ones are quite cheap. u might wana take a look at it)
2) Use back your USB modem, configure a pc connecting to the USB modem to be Internet sharing (ie. act as a gateway) but this will slows down the pc, should u used that pc for surfing also. You will need to get a Wireless Access point.
IMO, not worthwhile for option 2, might as well go for option 1. Go SLS jalan jalan, they have bundle offers on Wireless packages that comes with the router,pcmcia card etc.