How should I structure a History A level essay?

In my experience, the way an essay is structured can have an unexpectedly large impact on your grade. There are a few simple things you can do however, to ensure that your structure is as close to perfect as possible.

Introduction: in the very first sentence, you should state the stance you aim to take on the question. It is good to do this using phrases such as "I agree to a large extent," "I agree to a small extent," or "this is the case only to some extent." After you have made your stance clear, try and summarize each paragraph of your main body in one or two sentences. Then, close the introduction by restating the stance you plan to take on the question.

Main body: I would suggest 3-4 paragraphs each dealing with a different factor that proves your point.

For example, if the question was about how significant the Great Depression was in the Civil Rights Movement, each paragraph could deal with one of the impacts of the Depression and how this made it a significant event to the movement.
In this way, you can make your point clearly, as you will have something very specific to deal with in each paragraph. This also prevents you going off on needless rambles.

I would also highly recommend starting and ending each paragraph with a return to the question and your view on it. For example, it is extremely good to end a paragraph with something like "It is for these reasons I believe that the Depression was a significant event to a large extent." 

This will start to sound very repetitive but that doesn't matter, as you will gain marks for having a sustained argument.

Conclusion: your conclusion should basically be quite similar to your introduction. You should clarify your stance once more, and repeat the issues you dealt with throughout the essay very concisely.

I hope this was helpful. I can also send you some examples of essays that acheived close to full marks if you would find that helpful.