For GCSE and A-Level there is a somewhat similar structure to an answer, however it depends on the mark scheme and the Assessment Objectives which need to be covered.
For A-Level it is important to include an introduction, 3/4 main points which answer the question and present a coherent argument and finally a conclusion.
Every introduction should include 3 main things, brief context, the different points you will be making and finally your argument, which I will explain.
Firstly, it should include one brief sentence of context, so context around the question. For example if the question was 'Why did the Cold War end?' context could be given about when the Cold War actually ended, how long communism been around, e.g. "After over 70 years of communism, the USSR fell in 1991 with the collapse of the satellite states first and then the Soviet Union."
Secondly, after giving context you then need to write about what your essay is going to consist of. So, you need to talk about the different arguments which can be put forward, the different points you will be making. So, for same question as above, you would list the different reasons why communism fell which could be supported by a historian. For example, "There are many different interpretations as to why communism fell in the USSR. Dockmills argues that it was for economic reasons as to why communism fell, Ferguson argues that it was due to Reagan's harsh policies... ect.
Finally in your introduction you need to talk about you argument which is the most important bit. This mean what you think the answer to the question is, your opinion, and what you will be arguing throughout your essay. So for the same question you'd have to think what was the most important factor as why communism fell so for example... "However, the most convincing argument as to why communism fell, as argued by Jones, is that it was Gorbachev's policies as this allowed..." After stating your argument give one brief explanation as to why you believe this is the most convincing factor/argument.
Then you're ready to write your essay!