How should I write an introduction to an essay?

Your essay introduction should essentially condense your overall essay structure into one paragraph. While this may sound daunting if you haven't yet wrote your essay, a good tip is to avoid writing your essay introduction until the rest of the essay is complete. This way you know exactly what it is you're going to introduce.
Essay introductions ought to be brief; introduce the works or authors that are the primary focus of the essay first. Then write a sentence or two about what it is you're going to be discussing. Finally, it's also a great idea to write a sentence regarding your answer to the question (which should match your conclusion) as this will give the reader a clear sense of the essay's direction and act as an 'anchor' point.