Do a mix of everything:
Solving tactics puzzles on websites like chesstempo, lichess,... and many others is the easiest and best way. Make sure to understand the idea behind each puzzle, whether it is a fork a pin, etc. Chesstempo is good in this respect as it will show you (after you solve or after you fail to solve) the theme(s) of the puzzle.
At this point you should focus on simple topics like mating with king+queen, king+rook, basic pawn endgames. Lots of books written on endgames, but the basic ones you should be able to find online as well.
The importance of openings is often overestimated by beginners. I'd recommend not to focus on any specific opening, but instead learn and follow basic principles such as: occupy the center by pawns, develop pieces (bishop, knight) to active squares, don't move pieces twice in the opening, castle. Chances are that if you follow these guidelines you are going to follow (more or less) an "official opening". Don't learn any opening lines by heart so far. It is unlikely that your typical opponent is playing an opening for more than a couple of moves and also at this point in time you are not in a position to punish your opponent for deviating from the accepted best line, because you do not understand the ideas behind the openings yet.
Middlegame / developing a plan
This is the most difficult part. Once you stop blundering pieces in one move and falling into basic tactical traps, this is one of the most important ways to improve.
Finding a good plan requires lots of experience which you will gain through playing and analyzing your games. Essential to developing a plan is an evaluation of the current and of potential future positions. There are many factors that go into such evaluation, such as:
- material (learn about the value of the pieces); this is usually the most important factor
- activity of pieces (good pieces usually have lots of space to move and can attack enemy pieces; poor pieces are often hindered in movement by other pieces, particular by their own pawns)
- king safety (how many defenders, particularly pawns around your king)
- open lines (it is usually advantageous to occupy these by your rooks/queen)
- weaknesses, particularly isolated and/or doubled/tripled pawns can often be a good target for attack; also think of how you could create such weaknesses
Start by evaluating the position on the board for your and your opponent's side. This should give you ideas for a plan. For instance if you find that one of your pieces does not do much, think of ways to activate it. This could be for instance through moving the piece somewhere else, but also through opening lines using pawn moves. Likewise for the other factors. Did your opponent neglect king safety? Perhaps it is time for an attack.
Learning all this by yourself is very hard. Ideally you'd have a stronger player look through your (or other people's) games and discuss the evaluation of positions (and resulting plans) with you. If you don't have somebody to do that, second best would be to read annotated master games or watch commented games (chess24, youtube...) on the internet.
Playing / analyzing
Play lots of games at longer time controls (avoid blitz) giving you enough time to think about your moves. If you do this online (e.g. on lichess) you have your game automatically saved. Once you finished a game, run a computer analysis (can be done on lichess among others) on it which will point out major mistakes. Analyze the mistakes look at the recommended best move and try to avoid the same mistake next time you play. Repeat...