Just to clarify some things first: At your home, you might have say two computers, a phone, and a tablet. You think they are connected to the internet via WiFi, but they are not (usually): Somewhere in your home there is a router, which is connected to the internet through a phone cable or some other cable, and your computers, phone and tablet are connected via WiFi just to that router.
To hack into your WiFi, first someone must come close to your WiFi network. I mean physically close. The hacker could be in a car, not more than say fifty meters away. But they must be near you.
Your WiFi network should be encrypted (on your computer when the network is displayed there will be a padlock symbol or something similar). That means people including hackers can only get into your network if they know the password. So don't give that password to anyone.
Two precautions that you can take: 1. Never use http, but use https. For example, for this website enter https://security.stackexchange.com, not http://security.stackexchange.com. That means that everything will be encrypted, and the hacker cannot read it.
And use a VPN. Doing that means every internet access is encrypted (so the hacker can't read it), and it isn't sent to the internet directly but to the VPN server. So a hacker won't be able to see who you are talking to, they can only see that you are talking to the VPN server, and that information is useless to them. The VPN server will send your messages where you sent them to, but the hacker can't hack into the VPN server. The hacker can't read what you are sending because it is encrypted. And if you use https, then even if the hacker could decrypt the messages sent between you and the VPN server, they would still only get encrypted messages.