Enable wep or wpa and if people see your network it will say security enabled. If you don't want them to see it disable the 'ssid' but it doesn't really matter they can see it, they cannot access it. Next turn ON access control. Your PC is now the only machine that can connect. If you get another machine to connect you'll need to add it to the access list from the control PC first before it can connect.
Make sure you have changed the router access password.