beginner
endgames because with endgames you know how to steer a middle game, only really good players worry about openings because they have so much knowledge they are working backwards from endgames
Improving Your Chess - Resources for Beginners and Beyond...
https://www.chess.com/blog/RussBell/improving-your-chess-resources-for-beginners-and-beyond
"The most important fundamentals for the beginner-novice to study in chess are: Tactics, Endgame and Opening principles....while the last of these is certainly important to understand and to become skilled at, you can never study too much of the first two of these. That is, time spent studying and improving your tactical and endgame skills will accrue more to your chess success and rating than focusing primarily on learning specific openings."
You should learn most important principles of a chess game. That will give you a solid understanding of a chess game and you will know what to do in every position. Then, you should optimize your calculation so you don't miss tactics in your games. I can help you with all of this. Message me if you are interested.