Whenever you are writing a response to literature, you should first make sure to read the prompt question very carefully. Often, the question itself will contain hints on how to answer. For example, you may be asked to focus on a specific character, theme, or plot point.
To do this well, it helps to follow a structure. I like the one linked here from the University of Hawaii. It suggests that you start with an introduction, in which you briefly summarize the text and state your thesis, which basically means stating in clear language what you will be writing about. Next is the body. There, you lay out each of your points, all connected to the thesis, and support them with quotations from the text. (Quotations are very important! Always have the text in front of you, and take the time to prepare quotations you think you might use beforehand.) Last is the conclusion, in which you summarize the points you made and connect them once again to your stated thesis.
The short article on Leading Lights, which focuses on a young woman who scored highly on the IGCSE World Literature exam, has another suggested structure. It is called PEEL, which stands for point, evidence, explanation, and link. You can use it to write clear, well-organized paragraphs in your essay. You make a point, support it with evidence from the text, explain why the evidence matches your point, and then link it to the next point you want to make (or to the thesis).
I suggest that you practice writing with the PEEL structure. Take some time to find quotations, and practice using them in your essays. Good luck on the exam!