It's actually set in the router which forms your connection to the internet and ships the signal out to your computers. Some routers have the key printed on a label on the base if it's been preset, which as you don't know it, it may well have been. If that isn't the case, you need to cable to the router from your ethernet socket on the computer using a Category 5 cable that will have come with your router.
This is going to far more complicated than it look in writing so don't be put off. From your Start button, go to Run if XP - for Vista and Windows 7, use the Windows key and R together. In the Open box, please type the folowing. including all the spaces cmd /k ipconfig /all and then hit the Enter key. In the black form that follows will be an entry Default gateway. Make a note of that and type exit to come out of that box.
Open Internet Explorer and enter that address - something like 192.168.0.1 - into the URL address bar as though is was a web address. The router's interface will then open and present you with a login and password box. Most will let you in using admin as the login an dpassword as the password - post back if yours will not.
Once you're in, seek out the wireless section and then security. The WEP key will be there - either as ten or 26 characters and because it's probably Hexadecimal, anything that looks like an O is actually a zero and an I (for industry) is a figure one.
Keep a note of it outside the computer and in an easy to find Notepad document in your My Docuyments folder - call it NetworkKey.txt.