This is a very common type of question both for 'The Great Gatsby' and also other classics (taking a very obvious theme and asking you to present some interesting thoughts on it).
You're going to want to use the BEDOL method in planning for this question. Firstly Brainstorm all the ideas you possibly have on the idea of 'The American Dream' - there will be a lot and don't judge between relevant and unrelevant ideas just yet.
Now Evaluate four or five the ideas you have, which ones are pertinent to the question, which ones are key in understanding the text, which ones are just rubbish and you should probably not use? Next Develop these key points, you have to use evidence from the text and also don't be afraid to use some of the ideas you just discarded as they may be relevant now.
Ordering your points is now important, are there any points you need to make first? Are there some points which are not quite as strong and so should go at the end?
Lastly Link each point in your order to the point before and after. This can be very simple (using signposting phrases) or quite complicated and thematic - either way it will make the essay easier to read.
That's it, there's your plan sorted and all you have to do is write it (don't worry you've done the hardest bit!)