I wouldn’t worry about wayland vs xorg at this point. There are reasons to prefer one over the other but, as a new user, if it works it works. And if something is broken, it’s easy to switch between them (I assume it’s an option in the login screen?).
I’d just recommend whatever your distro defaults to, because that’s what they think works best.
Same as systemd if you stumble upon an argument about that at some point. It’s something the distro has made a decision about and taken care of, so it’s not something you have to choose.
As for a tip: On Linux, the “app store” (I think it’s called “Discover” in KDE?) is actually pretty good compared to Windows. If possible, applications should be downloaded from there rather than directly from websites.