The first step in any research process is to make sure you read your assignment carefully so that you understand what you are being asked to do. In addition to knowing how many sources you're expected to consult and what types of sources are relevant to your assignment, you should make sure you understand the role that sources should play in your paper.
For example, a common assignment at Harvard will ask you to test a theory by looking at that theory in relation to a text or series of texts. In this type of assignment, one source—i.e., the source that lays out the theory—will play a large role, as will the text or texts you're considering in relation to that theory. You may not be expected to consult any other sources. On the other hand, for an assignment that asks you to stake out a position for yourself in an ongoing debate, you may need to consult a number of sources to figure out the major positions in that debate before you can decide where you stand on the issue. For this assignment, you may also rely on sources to learn more about the context of the debate, to find the evidence that you will analyze to figure out where you stand on the issue, and to learn the definitions of relevant terms. Yet another assignment might ask you to formulate your own question on a broad topic and then answer that question. In this case, you will likely use sources in a number of different ways—as background information that will help you arrive at a question, as evidence and expert commentary that will help you answer that question, and as opposing views that you will take into consideration as you formulate your argument.
Many assignments will ask you to perform a combination of tasks, with the expectation that sources will play multiple roles in your paper and that you will make your own decisions about how to use at least some of your sources. If you don't understand what you're expected to do with the sources you find for a particular assignment, consult your instructor or a writing tutor. Understanding your instructor's expectations will help you to choose appropriate sources and to use them effectively in your paper.