Choose your topic if it has not been assigned to you. Choose something that interests you so you'll be excited about doing the necessary research. If you are interested in the topic, it will show through in your writing; equally, it's usually obvious when someone is bored with the topic. It doesn't have to be something you know a lot about already, just something you are interested in finding out more about and then sharing with your audience.
Sit down with your writing utensil and paper or your computer and work on a rough outline. The basic parts of an essay are the introduction, the body and the conclusion; the introduction and conclusion might be no more than a paragraph each, but the body of your essay will be the "meat," the sections in which you inform, report, argue and explain.
Draw in your audience's attention with an attention grabbing sentence or question, such as "Would you ever leave your car running while you run into a store?" This should get your reader thinking about whether she does or doesn't leave her car running and the reasons for the behavior. Go from general statements to more topic-specific statements as the paragraph progresses, and finally lead into your thesis statement, which tells the reader exactly what your essay will be about -- perhaps you are planning to show the reader why it's not a good idea to leave her car running when she runs into a store.
Use the paragraphs in the body of the essay to explain your thesis. Try to focus on one aspect or main fact per paragraph to keep your ideas organized, but don't just recite a series of facts. Keep it interesting by using many different sources of information, such as books, papers, newspaper articles or interviews. Perhaps you know a police officer who can tell you about cars being stolen when they are left running. However, don't generalize from one story or incident; use the stories to illustrate the facts and figures.
Draft your conclusion, in which you summarize what you've said. In this paragraph you will restate your thesis. You will want to end on an interesting note, perhaps with another question for self-examination or an appropriate quotation. Your goal is to leave your reader thinking about what you've said.
Gather your sources of information after you've finished your outline; as you find new facts, you can fill in the different paragraphs in the body of your essay. If the essay is about your personal experiences or opinions it may not require research, but you will still need to organize your ideas in a logical way, such as telling a story in chronological order or going from more general to more specific ideas. For example, in an essay telling a college why you want to attend, you might start with the fact that it's a beautiful campus or close to your home and then move on to say that the school has a great theater department and explain your interest in that program.