Statement learning backend is very vague, since there is a lot of different technologies to choose. I do not have your background exactly, however I'm backend developer, who switched between two technologies few years ago, but I hope You can still take something from my experience.
Personally I've switched from C# to ruby few years ago and I was delighted by how approachable this technology was for new comer. I've managed to setup simple app within minutes. I've started from rails for zombies but now it is probably heavy outdated. Than I've went through agile web development with rails It was good experience, because book is leading developer through project, and after each chapter one can see how big progress was made. And than I've simply started to do my own projects.
Edit: I've forgotten mention one thing. I highly recommend using linting tools as early as possible. In case of ruby you could use rubocop which will teach you correct styling of your code. And reek which will detect code smells in your code, and also provide tips how to deal with them.
Good luck :)
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