An overview of how to get started from scratch with Crystal
Crystal Official Docs: This is the go-to learning resource for Crystal at the moment. And they are pretty exhaustive, right from the basic literals and variables to some advanced stuff like macros. They also have a neat doc on coding conventions.
Crystal for Rubyists: A practical intro to Crystal language.
Crystal Playground - While Crystal doesn't have a built-in REPL (like Ruby, Python or Node), it does have a way to somewhat easily experiment with code. Just run crystal play and you should have a place to play around with Crystal in your browser at localhost:8080.
online editor - A web-based REPL for Crystal, it is good place to get a feel of the language. Without having to install anything.