Since you have asked this question under "Literature," I am going to focus on how to write an essay about literature, but the principles of essay-writing are the same for most kinds of essays.
You must begin with an introduction that includes a thesis statement, write a series of body paragraphs that support your thesis, and then write a conclusion to wrap it all up. That is the basic structure of an essay, but before we address each of those, we should talk about what needs to happen before you even begin writing.
When you are going to write an essay about a work of literature, a story, a poem, a novel, or a even a piece of non-fiction, you need to actually read the work of literature, fully and carefully. While you are reading, you need to think about what the author is trying to say, what important themes are in the story. You need to think about the characters and the setting, when and where the story takes place. Notice how people and places are described. Some or all of this will be important as you write your essay. When you sit down to write, you should have one main idea that you want to write about, an important idea that you have taken away after having read the story. For example, after I finished reading Paper Towns, I thought about how important it is for us to really get to know our friends, not just look at their superficial characteristics. When I read The Giver, it made me realize how important it is for people to make their own choices.
Once you have decided what that important idea is, it is time to write a draft of your introduction. Include the name and author of the literary work you are writing about. Tell the reader briefly about the story, who the characters are, perhaps, and the setting of the story. At the end of your introduction, you need to write a thesis statement. A thesis statement will state your main idea and let the reader know how you will support that idea. So, for example, if I were writing an essay about The Giver, I could end my introduction with a thesis statement like this:
In The Giver, Jonas learns how important it is to have choices, as he receives memories from the Giver, as he looks at how people in the Community have no choices, and as he finally decides to make his own important choice.
That thesis statement gives me the points I will make in my body paragraphs, so let's talk about body paragraphs next.
Your body paragraphs will each discuss just one point in your thesis statement. Using mine as an example, I will write a paragraph about the memories from the Giver, a paragraph about how the people in the Community have no choice, and a paragraph about the choice Jonas makes. I will write these paragraphs in that order because that is the order in which I have "listed" them in my thesis statement. For each body paragraph that you write, you need to begin with a topic sentence that tells the reader what the point is going to be in that paragraph. Then the rest of the paragraph should discuss the evidence from the story that is about that point. This is important, because each paragraph should focus on just that one point. Evidence can be direct quotes from the story or just a discussion of what happens in the story in your own words. Once you have written a body paragraph for each point, it is time to write a concluding paragraph.
The conclusion should be a review for the reader. Remind the reader what your main idea was, in different words from those you used in your thesis statement. Remind the reader what the points were that you made in your body paragraphs, a kind of overview of each, not with every little detail. This kind of review helps the reader remember and understand even better, just like when your teacher gives a little review at the end of a lesson.
Once you have a draft of an essay, it is good to go over it again, at least one more time, to think about whether or not you have good evidence to support your ideas, to make sure the paragraphs are organized nicely, and to proofread for grammar and punctuation errors.
No matter what kind of essay you are writing, remember the basic recipe, an introduction, a thesis statement, body paragraphs, and a conclusion. You can do this very well with that recipe!