The WEP key is also known as the Network key, it is an encryption key used to protect wireless networks from unauthorized use. WEP is the weakest form of network security but generally fine just to keep freeloaders off your network. You'll need to have that key in order to connect to the network.
To get the key, you'll need to open the router's interface page and look for it under Wireless Security. It should have either a 64 or 128 bit key in either HEX or ascii format, Hex will be either 10(64bit) or 26(128bit) characters long, an ascii key will be 5 or 13 characters respectively. The key may also be accompanied by a passphrase used to generate a set of keys. In this case only key 1(index) is needed to connect, but if your card's utility supports the passphrase feature, you can enter it and it will generate the key needed and connect.
Some Routers such as those from 2Wire, have a default network key already set in them, this key can be found on a label on the bottom of the router that is usually marked WEP and will have a 10 digit number in () or []
