Can you help me to write an essay about "Sonny's Blues"?

There are many aspect to this famous story that you could write an essay about. The idea is to choose one theme or idea and then use the text of the story to support your idea.  We will discuss a few ideas you could write about and then discuss how to organize an essay. 

One of the aspects of the book you might notice is the theme of darkness. Sonny's life is dark, the neighborhood he grows up in is dark, and the people are African-American, i.e., dark people. There is a great deal of evidence in the story to support that as a thesis.

Another theme of the story is the relationship between the brothers, the narrator and Sonny.  The narrator loves Sonny, but he disapproves of his life and his choices.  Sonny's love for the narrator seems much more unconditional.  That could be a good thesis to discuss the story, the narrator's attitude toward Sonny and Sonny's attitude toward the narrator.  They can be contrasted with evidence from the story.

Another interesting question to ask is why two brothers, raised in the same household, have turned out so differently.  There might be some answers to that in the story that you can discuss.

When you have decided what idea you want to write about in the essay, you can get started on your first paragraph, which should be an introduction.  Tell a little bit about the story, for the reader who doesn't know anything about it, give the author's name, and then write a thesis statement that lets your reader know your main idea and how you will support it.  For example, I could have a thesis statement like this:

"Sonny's Blues" is a dark story, dark in its people, its setting, and its mood. 

Your thesis statement is meant to help you organize your body paragraphs, one for each point you list in the thesis statement. So, if I were using that as a thesis statement, I would have a body paragraph on the people, a body paragraph on the setting, and a body paragraph on the mood.

For each body paragraph, I would have a topic sentence that lets the reader know which topic I am going to be focusing on.  And then the rest of that paragraph will include evidence from the story to show how this is a dark story because of one of those elements. Remember that in any essay, you need evidence to support your ideas, and in a literary analysis, this means evidence from the story.  

Finally, you will need a conclusion to wrap it all up. This should include a restatement of your main idea and the points you made to support that idea.