If all this is overwhelming to you, get a friend to help you or hire someone. Also, consider filing a lawsuit if you strongly believe this person is spying on you. This is illegal!
What can you do
Most likely the attacker got into your wireless network because you are using an old router with weak encryption or your chosen password is weak. He could have compromised devices inside your network afterwards, but that requires a more sophisticated attacker, making this scenario less likely. But not impossible! So here are the actions I would take:
- If he nested on one of your devices, it could be close to impossible to find out how he did it. Easiest choice might be to reinstall your operating system from scratch. Also, if you have more devices in your network than just your computer, you might have to reset these to factory settings as well. But to be honest I'm unexperienced how to deal with a compromised smart tv or similar.
- Note: This step is useless if your system is compromised. He can log your keystrokes and will know the new password! You should change the wireless password. If you don't know how, search online for the manual of your model. If you can't login, there's always a way to reset the device to factory settings, often by holding the reset button for a specified number of seconds. But be aware, that you'll then have to reconfigure the router yourself before you can use it again. You won't have wireless unless you finish the configuration.
- Note: This step is useless if your system is compromised. He can log your keystrokes and will know the new passwords! You should change your online login credentials (hotmail, facebook, google, netflix etc.). Specifically those for the sites that this person is referring to in your conversations. This might be a great time to start using a password manager like keepass. There are also plenty of online password managers if you fear to accidentally loser your password database.
Help us help you
We could help you a lot better if you could explain why you believe that someone is inside your network. So here are a bunch of starting points for you:
- What happened? When did it happen? What exactly did you observe?
- What Wireless router are you using?
- What wireless encryption are you using? You can find out by logging into the web interface of your router in case you did not know.
- What devices are connected to your network? Television, smart phone, tablet, home server, raspberry pi, notebook computer etc...
Regarding mutiple firewalls
Running multiple software firewalls will not improve your security. You mean it well, but it could worsen your overall security. Typically when you install a software firewall, it will deactivate your windows firewall automatically. Decide for yourself which one you want to use. The Windows firewall is a legitimate choice.
Next I'll address your question what someone can do once he enters your network. This is most likely not a complete list:
- The first thing to realize is, that this person now has the key to your wireless network. An eavesdropper can then read all your web traffic. Luckily HTTPS adds another layer of security such that your login credentials are encrypted between your computer and the server. There are however still websites out there which handle the implementation poorly or do not use HTTPS at all for the login form. In that case your unwelcome guest could obtain your login credentials. Even worse, if you are using the same credentials on other sites, he now has access to these as well if he finds out about them. This by the way is one of the reasons why you should use a different password for every website and maintain them with a password manager.
- Your devices are no longer protected by your router with NAT. If you are running local network services or have a NAS, Smart TV etc. in your private network, the attacker could now communicate directly with them. He or she could find security exploits and compromise your devices. From there on, depending on your setup, everything is possible.