I have to write a essay on four examples of greed in "The Rocking-Horse Winner" by D. H. Lawrence and need help structuring the essay.

I would spend the first paragraph introducing the story
just a little, mentioning that it focuses quite a bit on greed, and then at the end of
the first paragraph, writing what I call a "theme statement" about greed.  For example,
"In this story, Lawrence shows how greed is a dangerous force in people's lives."  That
will be the thesis statement that guides the rest of your
paper.

Then, in the next four paragraphs, structure it as
follows.  1.  A topic sentence where you state something like, "One instance of greed
being destructive in the story is..." and then, 2. List the example and explain it. 
Then, 3., discuss how that example is showing greed's dangers.  Follow that formula for
all of the paragraphs.  I'll include a sample below, to show how that will
look:

One instance of greed being destructive in the story
is when Lawrence describes how the mother of the family cares more about materialistic
matters than for her own children.  In the beginning of the story, it states that "she
could not love" her children, and yet was obsessed with luck and earning more money. 
Her lack of love drives her son to gain money, which leads to his death.  This obsession
with greed is one way that it is dangerous in the
story.

That paragraph above is a sample of how you might
write the 4 body paragraphs.  Then, in your conclusion paragraph, just summarize your
main points, restate your thesis sentence (the theme statement from the first
paragraph), and wrap it up.  I hope that those thoughts help you to get started!  Good
luck!