At A level it is best to keep the essay structure simple and clear for the examiner even though your analysis and thought processes should be more complex than GCSE level. Although each question will be slightly different, as a general rule I would recommend this structure:
You should start with an introductory sentence about the topic that should make clear to the examiner what the question is. Outline your argument/structure for the essay by saying something along the lines of: 'There are conflicting arguments regarding [insert topic] which fall under the main categories of [insert factors you will discuss in main body]. However the most influential factor was [insert the factor you will argue is the most important.]
Remember the introduction should be short, sweet and concise!
There is no set number of paragraphs but I would reccommend a minimum of 4 paragraphs but aim for 5! Write 2 paragraphs first regarding your chosen important factor, and then 1 paragraph for each of the other paragraphs. In terms of sentence structure it is a good idea to follow a PEAL (point, explain, analyse, link) structure to make sure you hit all of the marking criteria. The link is what most people forget in the essay structure but it is essential to a professional and focussed essay - examiners really go for it! Basically it means you link the point you have made directly back to the question and your overall argument (i.e. most important factor) before moving on to the next point. It is good to this at the end of each paragraph too.
Conclusions should (as the name suggests) conclude your essay so make sure you are not introducing new ideas or evidence. Instead, it should be further explanatory analysis of what you have already said in your essay! Make sure you are not repeating yourself, but try to justify your argument and how the evidence has supported this overall. Make sure your judgement is really clearly signposted to the marker and try not to 'sit on the fence' by not choosing one factor overall. It does however show a deep understanding to suggest that factors had a cummulative effect (as long as you maintain that one was the most influential!) or contributed to the impact of other factors etc. so the key is to show a relationship between factors rather than analysing them separately like you will have done in the main body of the essay.
There you go - a whistle stop guide to A level history essays! This is obviously just a guideline and is very brief so for further help structuring essays and arguments (or if you have any questions about this post) please do not hesitate to contact me for a tutorial.