The best way to protect your Wi-Fi is by changing your Wi-Fi password.

The ‘Wi-Fi network password’ is the password that you use to join the wireless network. Unless you’ve set your network to “No security”, you are already using a wi-fi network password.

If you’re using the router’s default password or a memorable one that you’ve set yourself, then you should consider replacing this password with a strong password. Remember, that we do not consider the router’s default password to be strong.

To upgrade the wi-fi network’s password, find the WIRELESS settings, usually located in the BASIC section.

Also, you can change the name of your Wi-Fi (SSD) to something other than the modem default.
Example: do not leave TPLINK2934S as the name of your wi-fi. People can know your router and possibly find loopholes easier or even see the default password (if you left the default)

Also, try not to use WEP encryption which is weak, and change your password periodically.