Secure your wireless router - most routers use a default password which can be easily guesses - like “password” for example. As soon as you set up your router you should change the password to something else. I suggest you use random letters and numbers say “asi8a7gahhg” or something similar, and write it down so you don’t forget it yourself.

Disable remote administration - most likely your wireless router has that feature and if you don’t use it you should turn it off.

Disable broadcasting of your SSID (Service Set IDentifier).

Enable WPA encryption. If your router (or any wireless device) supports only WEP and not WPA encryption you should enable it, although somewhat weaker than WPA it is still better than no encryption at all.

Minimize signal broadcasting - some wireless devices will let you reduce the strength of their signal, if the one you have is not one of them you should position it (if possible) somewhere in the center of your house (premises) - that way less of the signal will be broadcasted outside.