There seems to be this notion that beginners should avoid studying openings at all costs and tactics are the only thing that matters. This couldn't be further from the truth. A solid positional understanding behind chess is necessary to improve. Tactical ideas and motifs make up about 10% of the chess moves played in a game. The rest are positional moves meant in improve your position.
@Koo8384 said d5 against d4 and e5 against e4. Say your opponent does e4. Then you do e5. Then your opponent does Nf3. What now? You probably know what to do for at least a few moves, but say you didn't. If you didn't know any opening principles or ideas, you'd be stuck and might end up making a move like Bd6 which isn't the best. This is why it's necessary to understanding opening and positional ideas at least as much as you understand tactics. What baffles me is how much emphasis people place on beginners learning tactics above anything else. The thing is, if you play bad positional moves while your opponent plays good positional moves, your opponent will be the one with the tactics, not you. If you studied all your tactics but little to no positional ideas and strategy, you would be able to recognize all your opponents tactics, but soon they will start to overwhelm you and you will ask where you went wrong. The hard about positional ideas which makes it harder to learn that tactics is that you don't realize you messed up until many moves ahead when it is too late. You never understand the value of castling until your king is trapped in the center about to be mated. Here's what I recommend.
For openings, stick to solid openings-avoid gambits or systems. The problem with gambits is that you are generally going to give your opponent a better position (except for some like the Queen's Gambit) without your opponent having to do anything. The problem with systems is that you are doing the same moves over and over again and you won't really learn anything or improve. You might want to avoid openings with a lot of theory if you don't like memorizing. An example of an opening with a lot of theory is the Ruy Lopez.
Learn all the opening principles and master them. This is the standard stuff like castle, control the center, develop your pieces.
Then move on to middle game ideas and strategies. This is where knowing your opening becomes important. For example, in the King's Indian defense, black's middle game strategy is to push his kingside pawns and attack white's king. If you didn't know this, you would be lost trying to figure out what moves to play. Understand the different kinds of centers (dynamic and locked) and play towards that. If the center is open, move your pieces towards it. If the center is static, start pushing your pawns on the side where you have more space/ the pawn majority (the side where you have more space is in the direction where your pawns are pointing. For example, if you're white and have pawns on c2, d3, and e4, your pawns are pointing towards the upper right. Therefore, you want to push on the right side. Conversely, if your pawns are on f2 e3 and d4, then your pawns are pointing to the upper left and you want to push to the upper left. These strategical elements are imperative to learn and prevent you from reaching the middle game and wondering, "What now?".
For tactics do Lichess' puzzles. If you are ~1600+ in that you are probably good and won't let any silly mistakes like missing a fork slip by.
For endgames, I recommend studying the important ones like king and pawn and the basic checkmates like queen or rook.