Well, Stephen Crane is an author who is recognised as capturing the elements of Realism in his fiction, so you have picked a good author to focus on! If I were you, I would start by looking at the various aspects of Realism and then exploring how they are reflected in the various works of Stephen Crane. Obviously, his major novel is The Red Badge of Courage, which is widely regarded as a Realist classic, but at the same time I have recently enjoyed a number of his short stories that contain the same kind of themes, so you will probably want to look at a range of his literature and draw upon quotes from a variety of sources to prove your points.
Consider the following aspects of Realism: a rejection of the Romantic hero, a detailed focus on ordinary characters and realistic events, an emphasis on characters from working class backgrounds, and the use of dialect to reflect class structure.
Starting off with the first point, The Red Badge of Courage can be seen as a discussion of heroism and what it is to be a hero, as the protagonist starts off with a very Romantic notion of heroism and comes to reject this. All of Crane's fiction focuses on realistic events, and you can talk about the way that he accurately reflects the horror of war through the description of battle and injuries sutained by soldiers. The last two points can be covered by looking at "The Mystery of Heroism" by Stephen Crane, which features the sharply delineated structure of army ranks, and the dialect of the protagonist, who is a Private from the working class.