The topic you choose to explain in an expository essay should be something with which you are familiar. Therefore, research your topic so that you convey that you know what you are talking about when you are crafting your essay.
The topic you choose is not going to incite passionate debate nor will it polarize your readers. Therefore do not include your personal opinion of the topic when crafting your expository essay. Much as a journalist does (or should do), you are presenting a topic objectively and as free of bias as possible.
Include supporting facts about the topic you chose to write about for your expository essay. You should have found at least three when you were researching and creating an outline. Remember to take nothing for granted and assume your reader is completely unfamiliar with the topic about which you are writing. Much like a teacher would, be prepared to present the topic and support facts about it to your reader.
After you craft your expository essay, read it and ask yourself whether you have clarified the topic sufficiently in a manner that is clear and direct without talking down to your reader. Ensure there are no generalizations in the essay. The topic must be supported by facts and all problematic language or jargon appropriately explained.