Usually it's as simple as a setting in the router. I would check the manual for you and call out the page number but I can't (no model given.)
So in short, get to your router's setup screen and change the WiFi security to WPA2 and there should be your opportunity to set the WiFi password.
My wireless is not secured by a password and would like to change
that. But don't know how. I have an iMac. And my router is Netgear.