I cannot complete an assignment for you, but I can offer some ideas to get you started.
First, review the Gettysburg Address by Abraham Lincoln: a speech delivered by the President to provide a sense of hope for a nation divided against itself.
...the Gettysburg Address captured the spirit of a people seeking to maintain their unity in the face of divisive and destructive violence.
This address was made by President Lincoln at the location of the Battle of Gettysburg—one of the bloodiest on American soil in the nation's history—when it was turned into a "national war cemetery." Lincoln was clear that little of what the living did that day had any bearing on that spot in Gettysburg: what was done of any import had been carried out by those who fought there, and those who gave their lives at that place:
...we cannot dedicate—we cannot consecrate—we cannot hallow—this ground. The brave men, living and dead, who struggled here, have consecrated it, far above our poor power to add or detract. The world...can never forget what [the soldiers] did here.
In creating a scary story about the war, there are many options you can use in terms of your plot. There is fear associated with war. A young man going to war would be scared. For a family divided between the beliefs of the South and those of the North, another fear might be of facing—even killing—a relative: a father or brother. There would also be the fear of being captured by the enemy. A scary story might entail a young soldier, separated from his unit, hiding without food, water or shelter. These would be stories that could reflect those who fought at Gettysburg.
Your tale could be a ghost story, and could include a teenager (the protagonist) that goes to the cemetery on a dare and comes face to face with a young Union or Confederate soldier who walks the earth, unaware that he is dead. The conversation that exists between the teen and the long-dead soldier can convey experiences of the soldier, drawn from personal accounts of soldiers who survived that battle (that you could research). This would be informational and interesting. The response of the modern-day teen could speak to how things have changed since the war; as the writer, you could adopt a stance of whether the teen felt such a sacrifice was worth it, or that too much was lost.
To create a sense of fear, make sure to use vivid imagery, which includes sensory details. Imagery creates pictures in the reader's mind; sensory details appeal to one of the five senses: this would include hearing, seeing, touching, etc. The young man (woman) may hear the "skittering" of leaves across the lawn. The wind may be gently howling, making the hair on the back of his neck stand up. The image of the dead soldier may be so clear that the protagonist can see the weave of the material from which his uniform was made, perhaps dirt smudged across the soldier's face, or even a clean bullet hole through his jacket, just where his heart would have been. These kinds of details make your writing come alive.
In terms of the dialogue, you will want to refrain from repeating "he said." Use some variety: instead of "said," you can use mused (thought), sighed, whispered, insisted, declared, etc. Mixing your "conversational tags" up will keep your piece from becoming repetitive.
Organize the story's parts: introduction, body and conclusion. Stay focused on your topic. Use the setting to help create the mood.