How should I structure my essay answer?

There are two types of questions given by examiners: factors questions or yes/no questions. For instance, if looking at Hitler's rise to power you could get a question along the lines of: 'The economic situation in Weimar Germany was the main impetus for the rise of the Nazi party.' Discuss. This question can be identified as a factors question as it is asking you to discuss the different reasons the Nazi party gained popularity. In this case you must think of at least 3 other factors to discuss. In the case of the example these could be; Hitler's personal popularity; the failing democratic system and lack of real opposition; and political manoeuvring and lack of foresight on the part of politicians. Whichever way you conclude your answer, you should be discussing all of these factors with the predominant factor running as a thread through the essay. In other words, if you are concluding that it was economic hardship then while discussing lack of opposition as a factor you must also mention how either this was not as significant as the economy or how the dire economic situation contributed to the lack of opposition.
If the question were however: Was the economic situation in Weimar Germany the main impetus for the rise of the Nazi party? Then this can be identified as a yes/no question as the question is asking you not just to discuss factors but whether or not you agree with its statement. Similar points can be used in this style of question and similarly there should be a main thread running through your argument but every other factor given must be discussed in relation to the economy.
The best way to make sure you are fully answering the question is to use the PEEL paragraph structure. Start each new paragraph by introducing a new factor. Next give factual evidence, statistics, contemporary quotes etc. to back up your statement. Then delve into your evidence by explaining why it is relevant and how it proves your statement. Lastly, link each point back to the question. In the case of a factors question, why that factor is important/ not as important as the given factor, and in the case of a yes/no, why that point either agrees or disagrees with the question.
You should make sure that there is a clear thread of argument running the whole way through your essay. Your conclusion should not be a surprise to the examiner as your argument should be building throughout the essay. This will start in the introduction in which you should explain any key terms which need explanation or terms that you may be interpreting in a particular way, and then you should go on to introduce what line of argument your essay will take. Try to avoid the exam panic 'shove everything in' approach as this will not give you a strong answer. Take more time to plan if you need to and think about everything you know about the topic and how it is related to the question. If it is not related, don't put it in your essay. Your essay is meant to persuade the examiner of your argument so putting in irrelevant or erroneous details will just dilute the strength of your argument.
To really impress the examiner, try looking up the major historians who have written on the topic and try to summarise their arguments or, even better, learn a couple of quotes. Discussing what has already been argued within the historical community and how it either adds to your argument or how your argument disputes this is a great way to strengthen your argument. If you don't know any historians who have written on the topic then you could instead look up what the various historical schools have said on the topic such as the Marxist school or the revisionists and use this as a discussion point.