HOw do I strengthen my wi fi security?

You haven't indicated what your concern is or why, so all I can do is to offer the following. Virtually all modems and routers that provide Wi-Fi allow us to set passwords to access the Wi-Fi network. If you haven't done so, you can set a password. The security lies in the complexity of that password. If a person does not know it, then they cannot access the network. You can also set up your computer to automatically enter the password when connecting to it. If you have only one computer, then you can connect directly to it via an Ethernet cable and not even use the Wi-Fi.

Make sure your WiFi router and other networking gear has an admin password that you set. Do not forget the password you set. This prevents some web site from running javascript code that talks to your router and changes settings because you left the default admin password on the device.

If your broadband modem has an admin interface with a password, set a password for that.

if you have smart network switches (vs dumb switches you just plug in Ethernet cables into) make sure the admin interface has a password you set. You will know if you paid extra money for a smart Ethernet switch. Also very few people even by Ethernet switches, let along get smart ones.

Make sure the WiFi password in non-trivial. It does not have to be ugly, but it should not be the easy to guess 123456 or monkey variety. Just make it long enough, not a single word, and easy enough to enter so that you and your family can do it without making 20 mistakes along the way. For example "my mother the car". Do not use that, but something with multiple words, easy to remember by family members, but not something anyone would guess. "My Mother the Car" was a very old 1965 TV show staring Jerry Van Dyke, brother to Dick Van Dyke.

Your WiFi router should support at least WPA2 WiFi encryption. WPA2 has been around so long that unless you have very old WiFi hardware, you will have WPA2 encryption. If you do not, then maybe it is time to get new WiFi hardware.

A lot of users get their WiFi hardware as part of the ISP broadband package. If you hardware is so old it does not support WPA2, then get your ISP to replace the hardware. Many ISP will replace very old hardware if you ask them.

Look in your wifi router's user manual for how to secure the wifi.