An essay must have the following main features:
1. An introduction. In an introduction, you give the reader an idea of what your essay will be about. You should have a thesis statement that does this. If your essay is to be persuasive, you should tell the reader what point you will be trying to make.
2. A body. In this part of the essay, you give the evidence to support the point you are trying to make. Or, if you are just writing to convey information rather than to persuade, you cover the major information that needs to be presented to the reader.
3. A conclusion. This part of your essay should wrap up your argument. You can restate the gist of your argument, or you can suggest further topics for research, or you can issue a call for action -- it depends on what sort of essay you're writing.
The first link below has information on how to write various kinds of essays. Scroll down toward the bottom to find these.
The second link is to the essay writing group here on enotes. There's lots of information there about, no surprise, writing essays.