you could go here
http://www.linksysfix.com/check/HomePage/index.htm
click on netset and then there should be an option for security.
Or go to your routers setup page, click on the wireless tab, select security and change the mode to WEP, and enter a passcode. You also need to make sure your reciever knows the code
Tell me if that didn't help, and i'll put more detail in