Keep out WiFi Leechers?

What part of set up do you go to on your wireless router to keep wifi leechers from using your bandwidth?
I have a Linksys WRT54GS wireless-g broadband router with a Linksys WMP54GS wireless-g pci adapter.
Thanks for your time.

in my Linksys AP there is a tab Wireless where you can choose the encryption, SSID Broadcast etc in your Router Admin area look for something similar.