Plug a computer or laptop directly into the router using ethernet cable, then you will need to log into your router or wireless gateway by opening a web browser and typing in your device's IP address. It will immediately prompt you for the default user name and password for your network. This site has list of most router's default user name and passwords and ip addresses... http://adf.ly/idWPw
Once logged in you will then need to look for the wireless settings options and make sure to disable SSID, and also make sure WPS is disabled, that is a horrible security risk auto enabled by default. If you want to get uber techy you can google mac address filtering, however I personally will not drive down that tangent. :>