How do I write an effective introduction to an essay question?

In most cases, an essay question is asking you to debate a certain topic. For example, if you were to study Doctor Faustus, a question-title may be "Does Marlowe present Faustus as a character with whom we can sympathise?". A successful way of introducing this essay would be to briefly consider both sides of the argument, one side possibly being that Marlowe creates no sympathy for his character, as the play is rooted in traditional Christian values and Faustus is dragged to hell for his sins in the play's conclusion. The other side could be that Marlowe portrays Faustus as a tragic character, creating certain amounts of sympathy for him. You could also briefly introduce any scholars you intend to bring in later on, whose writings are relevant to this question; of course this is not a necessity.

At the same time, however, don't give your argument away in the introduction. Keep your opinion fairly ambiguous to begin with, as you point of view should become clear to the examiner over the course of the essay. This is also why a good introduction shouldn't be too long. Don't spend an age introducing the main points of the argument, and don't give a plot summary of the play. A common mistake is that people spend too long in the introduction and in the beginning of the essay rambling or giving their essay exposition. Launch into your argument as soon as is appropriate.