Your router will have a setup page with all the settings on it, usually accessed by typing it's address into your browser, something like 192.168.1.1 for example. To find out what yours is go to Start > Run, type in:
and click OK. Then in the the black box, type:
and click OK. You will see a "Default Gateway" entry, that is the address of your router's setup page. Type that into your browser and you will get a box asking you for a username and password. I think the defaults for Sky are "admin" and "admin" (without the quote marks) click OK and you should see all your router's settings. Look for the Wireless Security settings, your Security Password should be there.