For a more detailed explanation on why learning Rails is still a good choice even in 2018 you can read this article, but just keep in mind that despite all the articles about its decrease in popularity, it is still in the top 10 of the most used programming languages and frameworks. Studying alone can be very hard and if you want to achieve results you will have to put in a lot of effort and make sure that the resources you are using are the best you can find. That is why I'm writing this list including the best resources I've found after many hours of research.
I use Udemy a lot and it's rare to find a teacher as good as Jordan. Many teachers will just ask you to follow their steps but he actually goes the extra mile and explain a lot of what's happeping, how Rails works, why something is breaking and what are the best practices to follow. His course focuses on building a portfolio website and a blog and utilizes Rails 5. It's quite long and I suggest you to watch it at incremented speed because Jordan speaks a bit too slow. Apart from this minor flaw I think this is one of the best comprehensive courses on Rails that you can find. You can buy this course here and be sure to check the author's profile for more courses.
I believe in learning by doing projects and building AirBnb following Leo's videos will teach you many of the features you will probably implement on your future projects. Despite not being a native speaker, there are no problems in following and understanding him. Sometimes his explanation could be better, but as far as video courses that make you build a real product I believe that this one is the best. I remember struggling a lot to find ways to implement certain features on my personal projects and I have to say that this course helped me a lot to understand how to do them and what I was doing wrong. The course has been released in the summer of 2017 after a very successful Kickstarter campaign and it is updated to Rails 5. You can buy this course and many others here.
Earlier in 2017 I attended a 2 months rails bootcamp and I remember everybody in the class read this book (including me, of course). Why? It's very simple - it's free to read online - it's easy to follow - it provides you with all the foundation to build your own web applications If you know nothing about Rails and you just want to test the ground then this book is what you need, you can read it for free here but you can also buy a copy on the author website
The now concluded series of videos by Ryan Bates used to be the most popular resource to learn new tips and trick for Rails. Unfortunately, the last video dates back to 2013 but don't worry, Rails may have changed a lot but the way of implementing many features, and the logic behind is still similar. That is why, despite the old age, these videos still prove useful today and they will surely help you in your path to improve your skills
Although not as popular as railscast, Drifting Ruby and GoRails provide tons of useful tips on how to implement features and how to improve your app. I suggest you to check out both of these channels once you have a solid grasp of the basics, maybe after reading the aforementioned Ruby on Rails 5 book.
With the abovementioned resources you should have enough material to cover everything from the basics to an intermediate level and be able to create simple and advanced apps. Whenever you encounter a problem remember that stackoverflow is your best friend and if you can, try to join a community around your area, or one online because working alone can be very tiring and stressing. Seeking the help of other programmers will help you improve much faster than you think and will also make the learning experience more enjoyable. Happy Coding! and enjoy your time with Rails, I am sure you will love it as much as I do.