Which modem are u using?
You have to enable wi-fi on your modem and set up an access point through the modem configuration page.
After that just search for wi-fi networks on your laptop and after you see your access point, just connect to it.
It may be better for you if you call BSNL and ask them to send a technician to set up your broadband connection. When he comes, just ask him to set up the wi-fi connection.