Others have linked to the book collection, there you'll find walkthroughs of the basics, building web applications, tracking code in git, and so on. A lot of the common packages also have examples bundled that you can play around with and read documentation in.

Once one gets over the initial hurdle of the syntax, message passing and how to use the GUI to create new projects and objects one can usually just surf around in the image and look for examples to figure things out.