If you are piggybacking off of someone else's wireless signal, ask them for permission. If they grant it, they'll give you the key. If not, pay an ISP for service.
You can access the lan from a cmoputer physically connected to the router if you know the ip. Different companies use a standard IP- Link sys uses 192.168.1.1 as its standard IP and Ip's vary by company... if you can get to the router IP you can than access your routers information and look up your security key.
If you are piggybacking off of someone else's wireless signal, ask them for permission. If they grant it, they'll give you the key. If not, pay an ISP for service.