Sometimes it's printed on the label of the router. If it's not, you have to interface with the router through Internet Explorer and for that you need the address and to be cabled to the router. Use your Windows key and R together to bring up the Run box and in the Open box, type
how do i find my wep
cmd /k ipconfig /all
including the spaces. In the black form that follows, look for Default Gateway and note down the code beside it - 192.168.2.1 or similar.
Type exit to leave that form, and then open Internet Explorer. Type that code into the URL bar as though it was a website. When faced with the login screen, try admin as the login name and password or admin as the password. Post back if that part doesn't work. If you're in, look for Wireless and then Security. WEP will be part of it but you may have a higher level of security but seek out the Wireless Network Key and note it down. Input that when required and you should be able to take out your cable. Keep a note of that security key in the computer and another somewhere outside it.