Look at Samples and Models
Even within the same discipline, professors will have different expectations. For example, in sociology, one might ask you to write mainly about your own experience and reactions. Another might want you to do library or field research and disregard your own experience or attitudes.
Ask your professor to view a sample paper from a prior term or point you in the right direction to find journal examples and other professional publications. You can learn a lot about what is expected by what he or she has accepted in the past or views as a credible source material. Also, if you have written papers for this professor before, look at them again to remind yourself of his or her expectations.
As you look at the samples and models, take note of how various aspects such as outside sources, personal experience and critical analysis, section headings, language constructs and proofreading standards come into play.