Okay, fuck the rest of the advice you're getting - they're too vague.
GMs have two roles:>They decide when and how rules are applied>They provide the plot and the world for the players to play at
The rest of the things a GM should do come from these two, so these are the two things you must master.
To know when and how rules are applied, read the rulebook and have a reference of the rules at hand. Enough GMing will make it faster for you to resolve those rules, it takes time and to master the system. But this is pretty mechanic and most of the time this is more science than art.
The real challenge of a GM is the second point, providing the plot and the world. And this is what splits poor GMs from good ones. You might apply rules poorly, but if the game is fun players will mostly not care.
So, how do you do it? This is an art that requires experience and a certain philosophy of how to do it. What I would recommend at this point is to read narrative game rules, namely PbtA, Fate and Tenra Bansho Zero. They turn good GM practices into rules, so it's easier to get the hang of them. If I had to make a takeaway from them, it would be this:>Involve everyone on the plot, if you don't have how characters succeed or fail ask them how it goes and build from their suggestions>Focus on how to trouble the lives of the characters>Make sure the story is about the characters, not some meaningless third party thing>Fail forward. Maybe they succeed now but their success came at a major cost i.e. they actually shut down the magic trap on their roll but are now being chased by a dragon who caught them>If your system comes with a XP system, skew it and either award a set amount of XP to everyone or just hand the levels as the story goes forward; reward plot progression over mindless mob killing
I'd still read those books at the very least, and maybe play one or two games to get a handle of how the rules work. Then move on to your system of preference.