Most postgraduate modules provide you with a basic set of materials that form the core of your study. However, you will also have to find further information yourself, because part of an education at this level is developing the ability to determine the additional information you need, and the skills to locate and access it. It requires particular skills to be selective in what you read.
You will also need to read large quantities of text quickly, accurately and critically. In this context, 'critical' means discerning the strengths and the limitations of the work you are studying, so you can engage with it at an appropriate level.
Through your studies you acquire an understanding of the theories and concepts of your subject. In gaining an understanding, rather than simply 'learning' the theories, your thinking evolves and you are able to form well-reasoned judgments about the theory.
Your textbooks are your first source of information. They tell you the names of key theorists and you can also follow up on the references, the bibliography or further reading lists provided. By selecting those that are relevant and expanding your reading to them, you are led on to other relevant materials with their own lists of references.