The range of topics for essays is very broad, so it is hard to give specific guidelines. But some basics may be helpful. Your spelling and your grammar should be carefully checked. When you are making explicit reference to source material in the text of your essay, you should include a reference in the text: for example "Jones and Kenward (1990) conducted a study of college students and concluded ...", or "it is argued by Smith (1995) ...". The source articles that you directly refer to should all be included in the bibliography. The bibliography should be prepared in a standard format (there are several conventional ones) and each entry should include the title of the article or book, the authors, year of publication, journal name, volume and page numbers for a journal article, publisher for a book. When you are quoting or paraphrasing from a book or article, indicate that in the text either using quotation marks (for short quotes) or indented paragraphs (for long quotes).
Your essay should have an introductory section, explaining the background for your topic and introducing the material that follows, and a concluding section, summarizing the main points of your research, and/or drawing conclusions, and most likely two or more sections in between. The appropriate goal for many topics is to synthesize and summarize the current state of knowledge in an area, with reference to a few key sources. You don't need to be an expert on the topic, but you should play the role of an educated layperson. Some essay topics are more specifically directed to a particular thesis, and for these the writer is serving as an expert, trying to persuade the reader of the validity of the thesis. Most essays will I think be more effective if they are addressed to a potential readership of, for example, 199Y students (as opposed to be addressed directly to me).