Assuming you are interested in learning on your own, versus a more formal route like a code bootcamp:
Ruby on Rails Tutorial (3rd Ed.) is the gold standard. There are lots of other options, but this resource is what I and many professional Rails developers I know started with.
I’d recommend the Hartl tutorial for one reason— you actually write code on your computer. As great as the in-browser code schools are, there’s no substitute for installing the libraries, opening a text editor, and running a local server. That’s what developing in Rails is actually like, and the Hartl tutorial shows you how it’s done.