Your network administrator set it up when they set up your home network. You can't 'find it' unless you wrote it down.
If it's WEP, though, there's plenty of tutorials online that'll teach you how to crack the encryption. If you cannot find the key and nobody knows it, your router has a 'reset' button that you need to push with a paperclip or a pen or something that'll reset your router to all the defaults and you'll be able to set a new WEP/WPA passcode.
Also: sometimes the passphrase itself doesn't work when you're putting it into different computers with different NIC cards. If you encounter this problem, just put in the corresponding 64 or 128 bit Hex code and it should work perfectly.