Research and read. It is important to thoroughly research the topic being written about. Gather the sources needed to flush out the essay---some professors ask for a certain number of sources, so make sure to find them accordingly. Do not "cram" the information before writing the essay. Take time to understand the topic and all peripheral information that may help to understand it. Before writing the essay, take definitive, clear notes and rely on them as you begin the essay. Talk to the professor before writing the essay to clarify any points that you may not understand thoroughly.
Be sure to write the essay according to the professor's requirements if there are any. Some teachers prefer a certain amount of sources or a specific length. According to College Board, most essays should have a thesis statement. The thesis statement is usually a sentence or two that states an idea or hypothesis---the thesis requires support throughout the rest of the essay. The essay should provide information to provide evidence to the thesis. The statement should be supported by providing quotes and summaries of the text being studied. According to College Board, it is a good idea to reiterate and summarize the thesis at the end of the essay within the conclusion. If the essay does not require a thesis and only requires a description of an event or time, a loose thesis can be provided. The main idea should flow through the entire essay.
Always write the essay in a structured fashion, but pay attention to any format requirements set forth by the professor. Begin writing the essay by referring to your research. Start by providing evidence for the thesis statement. Many essays are written using five paragraphs---one for the introduction, three for the body and supporting details and one for the conclusion. Other essays are shorter. If the essay is shorter, write a short introduction, provide the evidence and details and then a shorter conclusion that sums up essay as a whole. Make sure to reiterate the points that the teacher taught.
Always make sure to proofread the essay to make sure it has no grammar, spelling or structure problems. Plenty of otherwise good essays suffer from poor grammar and sentence structure. Go beyond spell-checking. Re-read and have another set of eyes look at the paper, preferably the professor or a tutor.