It can be difficult to know where to start when you are faced with having to write an essay, so the most important thing is to ensure you have a plan. This does not have to be very complicated, but it should allow you to keep an idea of your points and argument as you write your essay.
The first thing to do is to identify what the question itself is asking you to answer; if you miss the point it will not matter how well written your essay is! There are three basic categories of questions: 'how', 'why', and 'to what extent' or argument questions. Often questions can seem convoluted and identifying how they expect you to answer can go a long way.
Once you have identified what the question is asking you to do you can come up with your points; you should aim for 4-5 depending on your writing speed. Different people find different styles of plan help, but I have always founds that spider diagrams were useful because they allow you to be flexible and to add or develop ideas as they come to you. If you are answering an argument question you should aim to have slightly more points for the side that you agree with, be that 3/4 or 3/5. At this stage it is good to drop reminders of quotes or key terms you would like to use in your essay so that you don't forget them later on!
When writing the essay itself you should first start with a good introduction. If you are doing a close anaylsis of a section of the novel then you should aim to place this in the wider sense of the novel before launching in to your anaylsis to show your awareness of the text. For other essays you should look to outline your points to show that you know what you are writing about and that you will have a sustained argument throughout.
Once you have written your introduction it is time to move on to the body of the essay. In order to structure your paragraphs many teachers like to use the 'PEER' or 'PEEL' method. This stands for Point Evidence Explain and Return or Link, and these are the elements you should aim to have in each paragraph. 'Point' means that the purpose of your argument should be clear from the opening line- the marker should have no doubt about what you are trying to argue. 'Evidence' is about backing this point up from the text; you should not be drawing your arguments from nowhere. 'Explain' means that you should then do some in depth analysis of the evidence you used and explain how it is linked to your point. 'Return' or 'link' is about linking the argument that you have just made to the question that you have been asked, and this helps to keep your essay focussed on the question you have been asked, not on all the knowledge that you have of your text.
Once your body is written all that is left to do is write your conclusion. In this you should aim to summarise your points and, if it is an argument question, to explain why you have reached the belief that you are expressing.
This method of planning and writing essays can be applied across subjects and even be continued well past GCSE level. It helps to give you a good structure to your essay and keep you focussed on the argument that you are making. I hope that this basic explaination has helped to give you a foundation upon which you can build up your knowledge.