If you've already set your router up, then it's as simple as logging into your router itself and looking up the WEP code. First, find the IP of your router. To do this, go into your network connections (Start >> Control Panel >> Network Connections). From there, you're going to have to find your active network connection. In Windows 7, click on the router's name, and then hit the "Details" button.
Now in the box that pops up, look for any numbers that might be your IP address. Usually, it looks something like 192.xxx.xx. Enter this number into your browser address bar.
Most routers require a username and password to gain access. Refer to your router manual, it should be in there. Once you've accessed your router, look around for a "Wireless Setting" tab. It should contain your WEP key.
If you still can't find it, let me know what router you have and what OS you're running, and I'll be able to help you more.