Openhab is a project like HomeAssistant. Both are basically websites that offer pre made smart home functionaility and can run on your own server. Openhab doesn't set a password by default (iirc), and when people expose it to the internet they end up with random bored people in another country somewhere flipping their lights on and off or adjusting their thermostat, though they could also get hacked. The openhab example was one of what not to do. I could have been more clear about that.

Security is an issue that people in this community are fairly opinionated on. Try to build up a practical knowledge of every tool you use (like tools for remotely managing your server). Think about how much access that tool gives you, and how easy it is to get. Ssh gets you basically full access to the system, except for bios level settings and things, but it is generally quite secure, and you can use keys instead of passwords. Cockpit, a remote management tool you can access from your browser, offers you a full terminal, so functionally the same access as ssh. However, hackers nearly got a back door into openssh (ssh is the protocol, openssh is the software on linux that implements that protocol), and cockpit is much less thoroughly looked at. Also cockpit doesn't let you use keys.

You're website will be static, which decreases the complexity and makes it easier to make it secure, so don't worry too much. Here are some links that might be useful: https://www.digitalocean.com/community/tutorials/how-to-configure-ssh-key-based-authentication-on-a-linux-server

https://www.cyberciti.biz/tips/linux-security.html (this one seems to be geared more towards enterprise stuff, so not all of it is relevant, but a decent amount is.)

Edit: This one is good too. I recommend at least skimming it before the one above this. It will help you figure out which of the points in the link above are worth paying much attention to. https://owasp.org/www-community/Threat_Modeling