How do I structure my essay and manage my time in the exam?

The best way to structure your essay is to understand exactly what the question is asking for. If the question is asking you to analyse the significance of a certain aspect of a text, then your first thought should be 'how is this aspect represented/handled?' and 'what does this mean for the text overall?'. Then, say the question refers to the significance of gothic tropes, you would proceed to locate these tropes and analyse how they are portrayed and what effect they have on the text overall. The points that you come up with provide the basis for your essay, and an allusion to them can form your introduction as well. Your conclusion gives you a chance to throw something new in, whether it's an idea demonstrating a different angle, or reference to a broader context or scholar. The final sentences of your essay can leave a lasting impression on the reader, but also cement your argument. Additionally, it is advisable to be aware of alternative sides of the argument and demonstrating your awareness of these counter-arguments can help to enrich the overall ambition of the essay.
Something to remember when writing an essay is to try not to include everything you know. You might have learnt a vast amount of knowledge on a particular topic, but when it comes to answering your exam question, tailor your knowledge and keep it simple. This makes it easier for the marker to follow and allows your argument to be coherent. Generally speaking, you can structure an essay under the headings: Introduction, Points 1, 2 & 3 (you might have more or less), and Conclusion. This structure is simple and by no means fixed, but can provide a good starting point from which to build your essay. Finally, when you do get into the exam, don't feel pressure to immediately start writing. Instead, take time to think and plan what you're going to say, and this will likely result in a more balanced, thoughtful response to the question.