I am serious about learning chess. Where should I start?

Analyze your games, both your moves and your opponents, it's time consuming but it helps. Tactics, tactics, tactics, cant stress that enough. And finally take your time, learn from your mistakes and most of all enjoy this great game! 

Varuzhan Akobian has some very good instructional lectures on youtube, he is easy to listen to and takes things step by step.

Lichess is a great place to study.

The Chessbrahs, in particular GM Hambleton have a Youtube series out that you should look at:

https://www.youtube.com/watch?v=p8pZbhjL-fQ&list=PL8N8j2e7RpPnpqbISqi1SJ9_wrnNU3rEm&index=1

In poker there are three ways to play pocket Jacks - and they all lose. 
Same for the well intentioned advice given on this forever topic by players with zero hands on teaching experience. 
People relay what they believe works for them or regurgitate worn cliches.

The 1st step - most important and usually disregarded, is to understand what type of learner a person is - how they can best benefit from all the available resources. 

There are five established learning styles: Visual, auditory, written, kinesthetic and multimodal. Kinesthetic learners have to do something to get it, while multimodal learners shift between different techniques.

Being objective about ones learning skills is no easy task- experimentation and patience is needed. 
Where to start? With yourself !

There is no evidence they exist but they're a good model for learning anyways.

Honestly, puzzles are the most important thing at the beginner level.

You can also try playing with beginner-level bots. I recommend Martin (250), Elani (400), and Aron (750). These three bots are free.

study the middlegame of grandmaster play. hth !

I do the lessons on chess.com and i lern a lot.