A format, or the structure of an essay, typically includes an introduction to the topic (an essay has to have a title), the body, and the conclusion.
An introduction states the purpose of the essay and introduces the topic and what you're going to write.
The body has a minimum of 2 paragraphs, each explaining different points. You can write more depending on word count.
The conclusion basically summarizes everything, and an opinion may be stated, depending on the essay you're writing.