In order to write an essay on the great depression, you need to find sources and examine the behavior of people and how they invested money with banks and stocks during the twenties, and then see how it caused the great depression to arise and how the great depression affected the population of the time.
You can continue this story with FDR and how he devised the New Plan to fight the great depression and help people. It is important here to find information about how he changed the supreme court, increased the power of the federal government, created job opportunities and social security, and in general how he managed to solve the depression.