This is an important question. In order to write a good essay, you need three key elements.
First, you need a thesis. This is the most important part of an essay. A thesis is a statement that you are arguing for. In other words, it is a debatable statement that you are seeking to prove. For example, if you are writing about To kill a Mockingbird, your thesis might be: courage is the main point. Not all agree. So, you need to argue for this argument.
Second, you need supporting points to substantiate your thesis. The stronger your reasons are, the stronger your thesis will be. Think of the body paragraphs as your main supporting arguments or evidence.
Finally, you need a conclusion. This is where you restate your thesis. Think of this section as the final words you want to reader with. In short, summarize.
In order to complete an essay, you need a basic plan or format. First you need to form or outline the main idea or concept you wish to write about. Then you need to write an introduction of your idea. Then the body of your essay must state the pros or cons of your idea with some supporting information (inferences or facts, or citing of studies or research findings). The last part is a conclusion that summarizes your ideas and recommendations or alternative and constructive solutions that support your basic idea or concept. Overall, during writing you should re-read, revise, and reconsider your thoughts that support your main idea or concept.