Even at GCSE, structure is really important so a marker or teacher reading your essay can clearly see the different parts of your argument and award you all the marks you deserve. The first part of every History essay should be the introduction. If there is any terminology in the question, this is a good place to demonstrate you understand the question by perhaps giving a short overview of the period or event, or giving an interesting fact or quote related to the question. A fact or quote can often be best as it grabs the reader's attention. You should also answer the question in the introduction, and exemplify which each of your factors or points are that you will write about.
Your paragraphs should each discuss a different factor relating to the question, or if it is a two factor debate, potentially two paragraphs related to each side. In answering the question 'which factor was most important', it is useful to place what you perceive to be the most important factor first. The conclusion should bring together your whole essay and the argument you have put forward. It should most importantly answer the question!! It could even be effectively nuanced, and draw together different factors to show that on a wider scale they influence each other and should not be so separated.