A thesis statement must state your main idea and supporting points. This is the key to structuring and developing your entire essay.
Having that main idea must come first. What is Fitzgerald saying about the impact of wealth in The Great Gatsby? One idea that comes to my mind is that wealth does not seem to make anyone very happy in the story. Another is that wealth is not really the American dream. Still another is that inherited wealth seems to affect people differently than wealth that is earned. Now, what kinds of support are there for each of these ideas?
The idea that wealth does not make people very happy can be supported by looking at the characters. You wouldn't necessarily have to discuss all of the characters, perhaps just a few, such as Gatsby and the Buchanans. So, a thesis statement could look like this:
In The Great Gatsby, it is clear that wealth does not bring happiness by examining the outcomes for three wealthy characters, Gatsby, Daisy Buchanan, and Tom Buchanan.
The theme that wealth is not the American dream can be supported by looking at the wealthy characters, too. Gatsby is quite wealthy, but for him, the dream is represented by Daisy, whom he dies trying to win, while Tom and Daisy, who are quite wealthy, do not exactly seem to be living the American dream. A thesis statement based upon this idea might be this:
Fitzgerald shows that there must be something more to the American dream than mere wealth, as the novel shows Gatsby continuing to strive for his goal of winning Daisy, while Tom and Daisy Buchanan, who have great wealth, are quite unhappy, individually and as a couple.
The third idea, that the impact of inherited wealth creates immoral behavior, is borne out with a contrast between those who inherit wealth and those who have earned their money. We have both present in this novel. Here is a thesis statement based upon that idea:
The impact of inherited wealth in The Great Gatsby suggests that characters who inherit their wealth, Tom and Daisy Buchanan, are more likely to behave immorally than the characters who have earned their money, Gatsby and Myrtle and John Wilson.
Once you have settled upon your main idea, choose supporting points. Use those supporting points to organize your essay, with one point for each body paragraph, paragraphs presented in the same order in which you present the points in the thesis statement.
Wealth is an important theme in the novel, and I know you can settle on a great thesis to develop your ideas.
For more summary/analysis of The Great Gatsby, check out this video: