If its a ZyXEL router swith it on and connect to it via wifi or cable. Type
http://192.168.1.1 in the Internet explorer bar at the top. If it asks for a username and password to log in try 1234 and 1234
Then go to wireless lan in the menu. Tick the box wireless lan, WEP encyption put to 128-bit WEP and then enter 13 character code and write it down. This is from memory because I binned my telefonica router as it kept stopping working. They should come preconfigured and security enabled and the security password is printed underneath the router. You can reset the router back to factory conditions by using a pen and pushing a small button for 10 seconds at the back of the router