There are three important points to keep in mind to write a great essay.
First, you need to have an idea. To put it another way, you need to have a thesis. This is by far the most important point. Once you have a thesis, you will be well on your way. A good thesis is debatable. It is an argument that you are trying to prove in your essay.
Second, after you have a thesis, you will need to have a few reasons why you think your thesis is a good one - supporting arguments. These will be the body paragraphs and pages for your paper. In these paragraphs you should also think of the counter-arguments of your readers and address them.
Finally, you will need to have a good conclusion. In this section, you should not introduce a new idea. Just summarize your findings.
Let me give you an example. If you are writing a paper on Oedipus Rex, then a thesis can be: Oedipus led to his own downfall. Your supporting point can be: (1) Oedipus is arrogant, (2) He refuses to see the truth even if it is right before him. The conclusion would just summarize your findings.