You probably wouldn't leave your front door open so anyone can come in, your car unlocked with the keys in the ignition happy for anyone to have a drive and yet, when you let your friends, your children's friends or others connect to your home network you are essentially doing just that.
We're Talking Wi-Fi here as that's how most of us connect up. We all (should) take precaution when we first sleep with someone in case they have a nasty virus and we should take precaution when letting someone connect to our networks for similar reasons. Letting even trusted friends or family connect up could see yourself getting nasty malware, ransomware, losing files on your network or even seeing your smart TV becoming the worlds most expensive brick.
Enable Guest Access
Nearly all Wi-Fi routers have the ability to have at least one guest network, sometimes up to 3 or more. Some offer better guest networks than others but at least using a guest network for visitors is better than giving them access to your own.
If you are able to set up a few guest networks, you might want to consider separating your kids from your main network or putting smart devices into their own network, that way if one is compromised the others stand a better chance of not being hit.
Each router interface is different so you'll need to do a bit of hunting around on yours to find the same settings. If you want more advance features, you will probably need to ditch the router supplied by your broadband provider but even the basic ones should at least support a guest network.
Here is how I set up guest access on my router.
Log in to your router
Look for Guest Network, it may be under the advanced options
On my router there are a few additional security features but those are covered later
Enable the Guest Network
Select the Network or SSID name (this is what people see when they browse for networks)
Set the security type to WPA or WPA2 Personal (if you have WPA always use this instead of WEP, if you have WPA2, use this instead of WPA. Never use WEP unless it is your only option).
Pick a good password (have a look here for some useful tips: http://www.datalossprevention.co.uk/single-post/2016/12/30/How-to-pick-a-good-password)
Once that is set up, you are good to go! Whenever a visitor asks if they can use your Wi-Fi, give them your Guest network ID and password.
If you are fortunate, your router may offer you some additional security features for your Wi-Fi networks. These apply to your normal and guest networks. The aim here is to block guests that you have no control over, essentially limiting them to Internet Access only.
If your router has the ability to enable or disable access to your local network, disable it (if it isn't available it is likely to be disabled by default).
If you use your router to have access to shared USB drive, disable that for the Guest network.
Network Isolation, this is a good and powerful feature. It means if you are connected to the same network as other machines, if you enable this feature the router will not allow your machine to see the others and visa versa. For your home network that could stop your music streaming but here, we want to isolate each guest device.
If you follow the above and set up your guest network, you will go a long way to protecting your own photos, files and systems should one of your visitors access a dodgy site or already has malware on their phone or laptop.
The isolation mode is extremely useful if you want to protect your own network too, if you don't need to access files on other machines or stream music from one device to another, it is worth turning on as this will help protect all devices on your home network. If you do stream music from one device to another, you could put those devices in a separate guest network with isolation turned off.
Nothing will protect you 100% but isolating your networks and your guests will greatly reduce the chance of something going wrong.