I just finished year 12 HSC (equivalent of year 13/GCSE) and we had very similar essay questions to answer. If it's anything like what we did, which was quite vigorous and difficult to master, then:
You need to make sure you have at least 2 thesis points developed around the main point which is the theme of ambition. You then need to incorporate both of your texts and discuss how your thesis points relate to ambition in each of them.
One thesis could be something like "ambition can be key to success or defeat", then you would link that to Macbeth, and the same with Great Expectations in another paragraph. Make sure you have several techniques (4-5 is a good number per paragraph/thesis point) and plenty of quotes/other evidence to back up what you're saying.
Also you need to underline your key texts, ensuring that you have spelt them the correct way, with capitals etc.
Good luck, if you're still unsure, just ask.