First of all, you need to figure out your topic. If it is not pre-assigned to you by your instructor, choose it yourself; mind that the topic name should clearly state the subject and/or the goal of your paper, so avoid any vagueness or ambiguity. To do this, you will need to narrow down your topic: starting from a discipline (for example, astronomy), move to its more specific areas (stars), gradually narrowing the range of problems and questions related to this area (a structure of a star), unless you get a specific subject (spectral analysis as a method of studying a star’s chemical composition).
Then you need to come up with a working thesis statement (one or two sentences expressing the main idea of your paper) and do research, paying attention to data and materials which prove your thesis statement. Write an outline and a draft of your future essay; keep in mind that a classic essay has a five-paragraph structure: an introduction, three main body paragraphs, and a conclusion. In the introduction, you must provide some background information on your topic in order to provide the readers of your essay with a context; smoothly transit to your thesis statement, and start writing the main body paragraphs. Each of them should contain one key point supporting your thesis statement. Usually, the structure of a main body paragraph looks like this: topic sentence (or a transition from the previous paragraph); the key point; data, statistics, observations, and other factual evidence supporting the key point; a link to the thesis statement; a transition to the next paragraph.
Finally, write a conclusion. It should briefly repeat your thesis statement, and the main arguments you made to prove it. Show how and why the discussed topic is important to your readers. Do not forget to format the cited sources properly.