If you haven’t played yet the RenJS tutorial, it’s a great demonstration of what you can achieve with RenJS, and how to achieve it easily.
If you already played it and you’re ready to start your own game, head up to the Quickstart.
If you’re a writer and need help with the syntax, you can find how every available script action works and see examples in the Actions Guide.
If you’re a designer and need to ajust how the game looks, check the GUI section to understand how you can achieve your prefered look and feel.
If you’re a programmer and want to extend or understand how RenJS works, the Engine section shows how the whole thing works.