absolutely lost - how to i get my MacBook to the newest operating system?

hi all

I know absolutely nothing about computers, operating systems, and so on. But I've now found out (bc slowly but steadily i cannot use applications like Spotify etc.) that my operating system is too old to even receive operating system updates from Apple. My Mac Book uses High Sierra 10.13.6, which is apparently too old to receive updates at this point (I've been to this “place” so many times where they are displayed—nothing).

What can I do to get my laptop working properly again? Apple Store? I found a website (Open Core Legacy Patcher), but unfortunately I don't really understand it – should I try it?

Thank u <3