An introduction should be about 200-300 words long. If your question includes a key term, for example, 'Explore how hopelessness is portrayed in "Streetcar Named Desire"', it is important to come up with a definition (in this case, hopelessness) so that the examiner knows that you understand its meaning and relevance to the play. After this, briefly summarize the key points that you want to talk about in the essay - this is really useful to refer back to, in case you get stuck when writing! Your final sentence should nicely sum up the introduction, and clearly indicate how you will go about answering the essay title.