The WEP key is a password that secures your wireless internet connection from people connecting to it that you don't want to use it. Once you connect to it the first time and put in the key then Windows will store the key so you don't have to enter it each time.
If you have a wireless router you can log into it assuming you know its IP address and password. If somebody else set this up then you may need to ask them. You can connect a network cable to the router to access it directly but will know the login information to do so.
If you are the only one using the router you can reset it back to the default configuration and reconfigure it from scratch. There should be a reset button on the back or on the bottom. You can look online and get the default password for the router to login and start the setup.