How do I write a good introduction to my essay in an exam?

There isn't one way to write a great introduction, so be flexible and allow your response to the specific question you've been asked inform how you go about introducing your argument. Whatever your approach, your introduction should tell the reader what to expect from your essay. You might start by introducing the main subject of the essay and why it is an important topic. Your introduction may also provide an outline of the key arguments you will present in your essay and how you are planning to answer the question. Always keep your focus on the question, and keep it snappy. Do not give broad, generalised background information on the wider topic. Instead, concentrate on what is relevant to answering the set question. You might want to acknowledge the challenges in answering the question to show the examiner that you are aware of the nuances and difficulties in coming to a conclusion.
In terms of length, an introduction usually makes up 5–10% of your whole essay but there is no absolute rule on this.