Once you have researched your topic, you should spend a lot of time structuring and organizing your essay. Make sure your essay has a clear introduction, research question/focus (i.e. what you will be investigating), body, and conclusion.
Literary devices or literary techniques are specific structures that writers often use to add meaning or create more compelling stories for the reader. Some common examples are metaphor, alliteration, hyperbole, and imagery. These techniques can give the reader a greater understanding and meaning of the writers intent. This website provides a complete list of literary devices and concise definitions of each.