Explain to students that an expository essay contains only facts. Depending on their grade level, students may or may not have had exposure to essays containing opinions. If they have not, give a general idea of what an essay with opinions is all about and how to avoid that in this instance.
Teach students the basic format to an expository essay. This is simply a presentation of several facts that relate to the topic -- such as a list of the presidents of the United States, or a list of facts and figures about the Titanic. An expository essay should be these facts listed in paragraph form, and should make sense in context.
Teach students what an introduction is. This paragraph at the start of an essay should give a general overview of what that essay is all about. List several things you might find in an introduction, such as a sentence about the topic, a story about the topic, or some tidbit of interesting information that relates to the topic at hand.
Show student how to create an introduction for their expository essay. Look at the general topic and do some preliminary research to have an idea of the facts they will present. Teach students to come up with a few sentences about the topic and include a hint towards the information they will include.
Remind students that they can revise their introduction after they have written the rest of the essay, so that the introduction might remain pertinent to the entire essay.