Since your essay has to do with foreshadowing, an effective thing to do would be to cite, first and foremost, one quote from the story which tells you (as the reader) that something bad was about to take place in the story.
The central idea of "A Good Man is Hard to Find" is that fate is inevitable and that we are all subjected to it. This being said, ask yourself what events happened in the story that shows how this is a true statement.
Start with the first line in the story:
The grandmother didn't want to go to Florida
Looking back after reading the story, you know that this is foreshadowing because it almost seems as if she knew that "something else" was going to happen, or was supposed to happen that day.
The fact that she did not want to go to Florida, and instead she wanted to go somewhere else, is effective for the author to include two things: a) establish that the grandmother was not willing to stay alone in Georgia for fear of this escaped convict called "The Misfit", b) use the presence of the grandmother in the car as the cause of the detour, and the cause of the accident that leads the family to its ultimate fate. Therefore, "not wanting to go to Florida" already causes three separate events all leading to the fatal encounter with The Misfit, and thus also leading to the end.
Aside from these three factors, there is also the second mention of The Misfit at Red Sammy's Barbecue. The description of the man as someone willing to rob a place even if it had only "2 cents" shows that The Misfit can do anything for whatever reason he wishes. The fear that the grandmother expresses so intensely in her discourse regarding this man seems almost as if she were, unconsciously, calling his name in an act of self-sabotage.
Finally, there is the detour that changes everything. Why detouring? Why listening to grandmother? Is it a wise thing to change a set path? There is something eerie and illogical when people make last minute choices. There is always the element of the unknown, of the "what if", and "what comes next". This Gothic factor gives the foreshadowing in "A Good Man is Hard to Find" a uniquely distinctive tone that works well in moving one event to the next.