One book that I can highly recommend to anybody who wants to improve is "The Seven Deadly Chess Sins", by Jonathan Rowson.
ISBN-10: 1901983366
ISBN-13: 978-1901983364
This one does not focus on technical chess skills, but more on the psychological aspects. The thinking process itself. This has helped me more than all of the opening books, endgame books, middlegame strategy books, et cetera combined.
Especially if you also play chess in OTB tournament situations I also recommend you watch the Chessbase DVD "Chess for Scoundrels" by Nigel Davies. It's what I would call applied chess psychology.
Of course that is not enough.
You'll need an opening repertoire which gives you at least one opening for White and one response each for Black against 1.e4, 1.d4, 1.c4, 1.Nf3, 1.b3 and maybe even 1.f4.
I can't really help you with that, because it's a matter of personal preference and individual taste.
I like 1.b3 as White, but can't really recommend it as the objectively best opening. I like the Colle-Zukertort and the London System. Carlsen lost a game to Karjakin in the WCC2016 playing the Colle-Zukertort as White so it may have become even more unpopular than it was before. And the London has a reputation for being boring.
You'll need some endgame skills.
If you speak german, I recommend Bernd Rosen, "Fit im Endspiel" for a very basic foundation. Anything by Karsten Müller is good and Mark Dvoretsky is perhaps the gold standard of endgame theory, but he is very advanced stuff (Elo 2000+).
Peter Lalic has an excellent YouTube channel:
www.youtube.com/user/DraganLalic/playlistsI highly recommend his playlist: P.L. Chess Endgame Course. 64 short videos, that will get you far without much effort.
Above all else, I recommend tactics training. The puzzles here on lichess are good, but
chesstempo.com is even better. Their endgame trainer is also fantastic for endgames with no more than 6 pieces. It seems very boring at first, but it did help me improve a lot.
The top most important thing however is to have fun and enjoy the game! Never let it become a chore. Improvement will take time with any method of learning. Don't get impatient and don't put yourself under too much pressure.
I can guarantee you that once you have gained rating and noticed an improvement, there will come setbacks. Your rating will begin to drop again and you'll lose games unnecessarily. This happens to all of us. Do not let that discourage you! That is what really separates the stronger players from the weaker ones. The ability to stand back up after a fall and to come back stronger.
I wish you the best of luck and most of all lots of fun playing and studying chess!
PS: Almost forgot: A very important part of learning is to analyze your lost games. Not just with an engine, but really trying to understand the positions, trying to find better ideas and plans than you did in the game. Take your time for this. I do this with all of my OTB tournament games and usually spend 5-7 hours on each analysis. Often I will annotated these games for later reference. So I can also use them to prepare for future games in the same openings or against the same opponents. With online games you won't be able to do that for every single game, but do it for a selected few that seem most relevant to you.
PPS: If you understand german I also recommend the following four Chessbase DVDs:
Michael Richter, Planfindung
Michael Richter, Variantenberechnung
Martin Weteschnik, Taktikmotive Erkennen!
Gisbert Jacoby, Gewinnen in der Eröffnung 1 - Eröffnungstaktik.