Once you understand the nature of your assignment, how do you know what sources to use?
If you are asked to do a close reading of a book such as "Things Fall Apart" by Chinua Achebe to investigate the different themes in the novel, your main source will be the primary source which is the work itself. You can read other articles that will help you understand the work and the themes presented but they are optional. For this assignment, you will need to focus on the work itself and shed light on the themes from your own reading.
But if you are writing a paper that compares "Things Fall Apart" to a classical tragedy and Okonkwo to a tragic hero, then you will need to investigate how tragedies such as Oedipus or even Shakespeare's tragedies may have inlfuenced "Things Fall Apart" and examine the characteristics of some of the tragic heroes in classical literature to illustrate the point. This requires a lot of research and for this, you will need to consult books, journal articles, or dissertations on the topic. You can find books by searching the library's catalog. You can get journal articles by searching the library's databases. You can find dissertations on your topic by searching the database Dissertations and Theses from Proquest.
While using sources in your paper you need to ask yourself whether the source you are using helps to answer your questions or thesis statement or is relevant to the point you are trying to make in your paper. How does it influence your thinking about the issue in question? Does it force you to think differently about your topic? Does it generate another question about the topic which you may be able to research further? Does it bring up an issue about your topic that you had never thought of? Is the source you are using from an authoritative figure in the field, someone who is considered an expert in the field? If you can answer one or more of these questions your source is undoubtedly useful and relevant to your research.