An essay must have an introductory paragraph, body paragraphs, and a conclusion. Basically, in introduction, you're introducing your thesis or your topic. Also, this is where you make your standing point(s). Make it interesting, because if your introductory paragraph is weak, then the reader may not be too engrossed in your essay.
The body. The most important part of the essay. This is where you include your points (hopefully, with many ideas to back you up). The body generally consists of three paragraphs or more. Each paragraph can have many sentences, but make sure it's not less than three.
Conclusion. This is where you sum up everything that you have written so far. Make sure to end the conclusion with a strong statement. Alternatively, you could ask the reader a question pertaining to your standing point.