See what is actually filling your disk
Storage settings break the disk down app by app, and explain that mysterious System Data entry.
Tips tagged #settings
Stop the desktop appearing every time you click the wallpaper
By default a click on any visible scrap of wallpaper sweeps all your windows aside. You can restrict it.
Set up the Dock the way you want it
Hide it when you are not using it, keep only the apps you open, and stop minimised windows piling up on the right.
Force quit an app that has stopped responding
The Mac equivalent of Ctrl+Alt+Delete for a single frozen app.
Set up a Time Machine backup
Point macOS at an external disk once and it keeps hourly versions of everything by itself.