If you are just getting started, we recommend you begin with the OpenGL in Cinder guide. If you'd like to just get a sense of what's in Cinder, take a look at the Categories list. And don't forget, the samples are still the best way to learn Cinder for now as we work to fill out the documentation.
Cinder ships with numerous samples which are designed to demonstrate specific features of the library. Studying these samples is one one of the most direct routes for learning Cinder. A few examples: