Do your research. The entire point of an essay is to display the conclusions of the research you have done into the specified subject area. As a consequence, thorough research is imperative to completing a good essay. If the essay has to be completed for a course of study, you should have access to the appropriate sources, and if you don't have that source of information, many journal articles can be found online. If you cannot access the required texts, try a local library.
Plan your argument. Analyze the texts you have read, evaluating how they deal with the subject area, what they say and how effective their arguments are. If you have a set question, you should come up with a probable answer to that question before you start, and then test to see if your theory is correct. Your essay is essentially a long argument that you have constructed, based on your study of the resources, which addresses the question posed about your subject area.
Structure your essay. If you have several points to back up your argument, you have to discuss them in a logical order. Your points should clearly flow on from one another, building point upon point to construct your full argument. Essentially, you have to lay clear groundwork, so that anybody reading your essay knows exactly how you reached your stated conclusion. Essays aren't places to purge all of the knowledge you have gained; they are a forum in which to display an argument.
Write your introduction. An introduction to an essay should introduce the question, explain the issues in addressing it and basically state your line of argument. The introduction is literally written to introduce the reader to the subject and how you intend to tackle it.
Make your points. After you have introduced your essay, work through each of your points, step by step, remembering to cite any appropriate sources on the way. In each paragraph you should make a point, show an example and then explain how your example supports your stated point. This rule is a basic for writing essays, and it should be used as a guideline for each of your paragraphs, but it isn't a definitive rule.
Conclude your essay. In your conclusion, you have to wrap up your arguments, explaining how the points that you have made support your argument. Basically, a conclusion is a way of looking back at the points you have made and forming them into a succinct argument. Show how the texts and the points you have raised combine to support or to discredit the chosen line of argument.