Nigel has not quite got that right.
Security keys are needed to access Wireless networks that have been protected by encryption. Some ISPs supply routers with default keys set but one is always counseled to reset them to one of one's own choosing.
So to answer the question, you have to ask the owner of the network for the key.
In the United Kingdom, using someone's network without their consent is an offence.
t, August 2008