Who is the protagonist and the who is the antagonist and why?

A protagonist is the leading character of a story. An antagonist is this character's opposition or adversary. (Even so, an antagonist is not always a morally bad character. For instance, Detective Fix in Around the World in 80 Days is a good character with good motivations. Still, he opposes the main character's goal of getting around the world as quickly as possible; he tries to slow him down. Therefore, he is still the antagonist.)
In this story, the protagonist is Yellow Sky's Sheriff, Mr. Potter. In the story, he has just gotten married in San Antonio and is returning to his house with his new wife. When he arrives, Scratchy Wilson, a town drunk, is threatening everyone around him and trying to convince somebody to fight him:

"There was no offer of fight—no offer of fight. The man called to the sky. There were no answers. He screamed and shouted and waved his guns here and everywhere."

Then, he remembers his friend, Sheriff Potter, who he often has gun fights with:

"But still there was no offer of fight. The name of Jack Potter, his ancient enemy, entered his mind. He decided that it would be a good thing if he went to Potter’s house, and by shooting at it make him come out and fight. He moved in the direction of his desire, singing some sort of war song."

He goes to Potter's house with his guns. Sheriff Potter has just arrived in town and is not carrying a gun. When he arrives, Sheriff Potter (the protagonist and central character of the story) tries to convince Scratchy Wilson (the antagonist, who is threatening the Sheriff's life) to leave him alone. He tells him he has no gun and cannot fight him. Scratchy Wilson doesn't believe him and continues to threaten Potter:

“Don’t you tell me you haven’t got a gun with you, you dog. Don’t tell me a lie like that. There isn’t a man in Texas who ever saw you without a gun. Don’t think I’m a kid.”

Finally, Potter tells him that he isn't carrying a gun because he just got married. This news leaves Scratchy Wilson entirely surprised (since Potter hadn't told anyone in the community that he was getting married). Potter's surprise marriage drastically affects the conclusion of the story.