How do i find the security key or pas...

Legally, you don't. Routers have a password reset method to log into them, then you can check out / change the password for the network.
Assuming you're talking about a wireless network, it depends on the encryption. WEP encryptions can be cracked with injection soft/hardware... I know there are a couple of apps that do that in linux. For WAP encryption, good luck. :)