Most expository essays require that you take a research-based stance on a topic of wide discussion, like the effect of education on poverty or the primary weaknesses of the Confederate Army in the Civil War. For this reason, you need to conduct research on the topic you are writing about so that you can support your argument. In some cases, this research might be the studying you have been doing in your class. Your teacher will then expect you to cite relevant facts and arguments from the required and optional reading in your essay. Your teacher might also ask that you do outside research, citing academic journal articles, books and websites. As you conduct your research, consider the question you've been asked and use the research to help you form an opinion. Keep a list of those resources that will help you support that opinion, as well as the opposition's primary arguments so that you may refute them. If you do not have access to the question you will be asked to answer, such as is the case with most essay exams, use your research time to gain an understanding of the debates scholars typically address regarding the topic you are studying.
If your research hasn't given you a clear idea for an argument, start with pre-writing. Free write, or write about the topic for a specific amount of time without stopping. Play word association games or make an idea map to help you come up with something to write about. Once you have your argument, craft a thesis. According to the Purdue OWL, without a well-written thesis, you will have trouble writing an effective expository essay. Your thesis is a one-sentence statement of your argument, and it should include a topic, a claim and a "because" statement that lists your support points. You may start with just a topic and a claim, and then do some more pre-writing to come up with those support points. Further, your support points may change as you begin to craft your essay. When this happens, make sure to go back and modify your thesis so that it reflects the proper support points. Once you have written your thesis statement, create an outline that will structure the order of the rest of your essay. The outline should include major support points and the minor points that you will use to explain and describe them.
According to the Purdue OWL, many people choose the five-paragraph essay structure to write an expository essay. This structure requires an introduction, three body paragraphs of support (which correlate to your support points in the thesis) and a conclusion. In other words, this essay has a beginning where you introduce the question, a middle where you explain and prove your argument and an end in which you discuss the significance of your argument based on the information given. Because most expository essays are in response to a specific question from an instructor, you must be sure to structure your essay in a way that allows you to answer the question given. For instance, you might be asked to compare and contrast two historical events or analyze the causes of a war. In the first instance, you might have just two body paragraphs: one of comparison and one of contrast. In the second, you might have four if you have identified four major causes of the war. Do not be afraid to be creative and innovative, but make sure you follow the rules set forth by your instructor.
Because an expository essay is, essentially, an argument, it's important that you use good strategies of argument. For instance, make sure the evidence you've given to support your claim is, indeed, evidence and you are not committing any fallacies. There are a number of fallacies, so review them before you write. Also, refute the opposition's primary arguments in addition to proving your opinion in each of your three body paragraphs. Finally, the best arguments are composed of appeals to logic (logos), credibility (ethos) and emotion (pathos). For an expository essay, however, rely most heavily on your appeals to logos, while appealing to ethos through the credible sources you mention.