ZOMG best question ever! btw.. how come they tell you what the question is in advance?
ANYWAY!
Firstly you should identify the quote, recall in the back of your mind who is saying this, because somewhere in your essay you have to acknowledge this quote. Also keep in mind the question type you're looking at. It is a 'discuss' question as opposed to a 'do you agree?' question, so treat the question in that manner.
You should also probably nut out the term 'atonement' because you will need to define it in your contention. This will help you to form your topic sentences. By all means you can look at characters and see how they all 'atone' for their sins in different ways, but be cautious of organising your paragraphs into characters, this is a very big no-no and will look too 'proper' and lacking depth in your responses.
Personally, to get a really sophisticated response out of this question, I would still brainstorm many of the characters and what sins they've 'committed'. What nature are they? Do some seem more extreme on the continuum than others? Then to get a bit more depth to your essay, see what the consequences are for when these characters - I'm thinking highlight Michael, Anna & Elinor especially - admit defeat and 'wrong doings'. What is Brooks trying to say about right and wrong? Keep in mind the era of this novel, and have that at the back of your mind so that you don't make any absolute statements or compare our world to theirs. It's important to delve into the book and try to empathise with the characters even if their experiences are so far removed from our world.
Once you've done this brainstorming part, come up with 4-5 topic sentences. Have a bank of about 10-20 quotes (depending on how good your memory is) but not extremely long ones - some that you can incorporate and alter using correct methodology into your essay seamlessly. Have all this down on your plan, then begin to write. BUTTTT do not underestimate the value of brainstorming at home! This counts as STUDYING i.e. however many hours that these education specialistics are expecting you to do in year 12 for private revision and all that jazz. You can't start writing an essay from scratch, so set aside some time (daily, weekly, depending on how close your English assessments are) to brainstorm. Your opinions can always be "correct" so long as you justify your responses , so don't be scared to sit and reflect.