Unless you're taking a creative writing course, you probably won't be asked to write many fictional essays, although they do exist. Fiction is all about being creative, descriptive, and imaginative, so the most important part of writing fiction is to flex those storytelling muscles.
The difference between an essay and fiction is in the definition and what counts as an essay versus what counts as fiction.
A fictional piece of writing is, by definition, a made-up story. Good fiction takes imagination and creativity, which can be used to write essays or any other type of fictional writing. If you pick up a book, essay, paper, or read a novel online and it falls under the category of "fiction," what you're reading is not a true account.
An essay is a short, informative piece of writing that consists of an introductory paragraph, three or more body paragraphs, and a conclusion. Essays can be descriptive, expository, persuasive, or narrative. A writer only has the option to write fiction under the category of a narrative essay. Many narrative essays are true accounts from a writer's lived experience, but some are fictional accounts.
For an essay, certain requirements apply for each type that separate it from other fictional writings. While a fictional poem can be nearly any length and structured in any way that the writer feels is best used to express themselves, an essay should be short (around five to seven paragraphs including the introduction and conclusion) and include a thesis statement that lays out the point of the essay.
Nearly any kind of writing can be written as fiction, although doing so with some forms can get you into trouble. You don't want your thesis in college or the personal statement you submit to potential employers along with your resume to be anything but true, for instance.
However, fiction is a popular genre that includes many mediums. Novels, magazine articles, children's books, movie and television scripts, and short stories can all be fiction. Any story, even ones based on a true story that add details for dramatic effect, fall under the category of fiction.
Whether you're writing a fictional essay or any other type of fiction, have fun with and get creative. After all, that's what good fiction is all about.