The best introduction I have seen to date is the RailsTutorial Web site/book/screencast series: https://www.railstutorial.org
The book and training exercise are free to use on line, although it would be good of you to reward the author for his continuing hard work on this topic. This is a clearly written and very well paced exercise, which will acquaint you with the basic and useful corners of the framework, and more importantly, show you how to find out more as you gain experience and confidence.
After that, you may find something like The Rails Way or Ruby for Rails (now renamed The Well-grounded Rubyist) a great level-up.
