How do I go about improving the structure and content of my essays?

Good question! The first place to start is to read through your marking criteria long before you start essay writing (and ideally early on in the year!) - this is what your examiner will have and will mean even before you start you're targeting them. 

Now when it comes to the moment of starting to write, read your question and underline the key words. Work out what the question is asking: is about themes? Characters? Form? Once you've got that pinned down, think about trying to answer the question in ONE SENTENCE. This will form the backbone of your plan. From this, develop 3-4 body paragraphs, making sure that you include everything you looked at on the marking criteria. 

Once you've got a good plan (which should take roughly a quarter of the time you have for the exam), then start writing! Keep focused, follow your plan and above all ANSWER the question.