For me the first thing is to control your emotions.
Learn to fold when you have to, learn to get the most out of your hand when you have to.
So controlling your bankroll is very important.
It is also very important to make yourself a profitable player. learn to see in which game situations (tournaments, cash, sit&go. etc) you move better and get more profitability from your game.
Another important thing is not to become a gambler, this is a very serious mistake that has even happened to players who, after winning big prizes in tournaments, wanted to start their Pro career and in many cases ended up worse than ruined.
Persistence, study of the game, considering that there is also an important part of luck in the game, that also affects a good player. If you don't understand badbeats you can never be a good player.
once you become a pro you must take the game as a daily job.
My regards, brother