Expository is a fancy word for explanatory. In an expository essay, you are explaining something to the reader.
The structure of an expository essay depends on your topic. If you are writing instructions about how to do something, the best structure is chronological order. You start with the first step and then continue with the following steps. It's always a good idea to write a creative beginning and ending, something that will catch the reader's attention.
If you are explaining a different type of topic, then you have to decide the best way to organize your ideas. Ask yourself, why am I writing about this? (For school is not a helpful answer.) What are the most important things for the reader to know? If you boil your topic down to a few major ideas, then organize your paragraphs around those ideas.
Always write a short introduction letting the reader know what's coming. Try to be creative and original. It doesn't have to be long. Perhaps hint at the interesting things ahead.
The conclusion also should be brief, perhaps just a sentence. For example, who would think...