Simple, go to your internet browser, (this doesn't work with google chrome) and enter 192.168.1.1 in the address bar. It's a common ip address for most routers, if this doesnt work, heres a list of other default Ip adresses. http://compnetworking.about.com/od/workingwithipaddresses/f/getrouteripaddr.htm
If your prompted for a user name/password and either don't know it or never had one, try entering admin/admin. You might have to reset your router for this to work.
Navigate to the wireless tab and you should see it. If your WEP key is encrypted, just change it to whatever. If you have windows 7, you can right click your network and select properties. Click on security tab and make sure the "show characters" box is ticked. You should see your WEP key there.
You find it.
You search for it, if you have anyone in your house, see if they have the password.