The Guide tab provides a good starting point for beginners, in particular the topics under the "About the API", "Work with the API" and "Tutorials" headings.
The API Reference tab, as with Google's version of the same, is the place to go for detailed information on the various object classes built into the API including their constructors, properties, methods and events. There is also usually example code on these pages to help clarify usage along with links to samples that use the classes. I encourage you to follow along with the code discussion in the lesson by looking up the various classes utilized in the API Reference. Note that the classes are broken up under a number of headings including esri, geometry, layers, symbols, etc. If you have any trouble finding a class, you can always look it up through the Search box in the upper right of the page.
Finally, the Sample Code tab provides access to a gallery of sample maps that demonstrate a variety of classes and functionality. The samples are categorized on the left side of the page (e.g., Graphics, Query and Select, and Renderers). Clicking on a sample in the gallery pulls up a page that provides links to a live version of the sample and a zipped copy of the source code. The page also lists and discusses the source code. It is also possible to open the sample in "the sandbox," a tool for making modifications to a sample and seeing the resulting changes side by side. As with classes in the API Reference, you can look for samples that make use of a class through the Search box in the upper right of the page.