How can I structure my essays more clearly and without going off topic?

The first step is to understand the question and then create a good plan. You won't have long in the exam, so I would recommend that you practice writing plans using past papers as well as doing timed essays. Your plan should be structured using brief key words in the paragraph headings, and then bullet points underneath setting out your main ideas and quotes.
In the actual essay, your introduction should identify the key themes at the core of the essay question. If the question involves a quotation, make sure you respond to it. Outline what your overall argument or stance will be.
Start each paragraph with a concise sentence that makes it clear what you are about to say. In exam conditions you will probably be able to manage 3-4 paragraphs. Rather than making vague points, stick to using evidence such as quotes from the text. Be sure to refer to critical arguments and context as well. Repeat this point and analysis structure throughout your paragraph to develop your argument. Practice using sophisticated conjunctions such as 'moreover' or 'nevertheless' to develop your argument smoothly. At the end of the paragraph draw the various strands of argument together. Repeat this for three paragraphs.
Your conclusion ought to be a slightly more nuanced version of the introduction, drawing upon the complexities of the question while indicating your viewpoint.  You can use a quotation if it helps, but try to keep the whole thing brief rather than adding new or unrelated points into the conclusion