here's what you need to do,
first locate the default gateway ip of your wifi router, (192.168.x.x), this is your wifi settings gui address of your wifi.
2nd type the ip on a browser and the authentication user and password will appear.
3rd type the default user name and password (username: admin ; password: admin / password).
4th locate the wifi settings on the gui page settings of your wifi, then there is a wifi network setup on the menu.
5th you will ask to put an SSID (the name of the wifi that will appear on the signal) and a passphrase (password to connect).
6th after the settings reboot your wifi router.
most of the wifi you can detect the MAC address of each machine that are connected in your wifi router, meron ding mga MAc filtering and parental control on whom machine are you going to grant an access even though they knew your SSID or passphrase.