There are a number of good ways you can start an essay. However, it is always a good idea in your introduction to reword the essay title that has been given in order to make it clear to the marker that the essay is on task and on focus. This is such an important thing to remember when answering any essay. Although it is not clear from this question what precisely the essay title is, the introduction could begin in the following way:
In Flannery O'Connor's "A Good Man is Hard to Find," foreshadowing is used to both highlight the humour of the story but also point towards its tragic end.
This would need to be developed, of course, depending on the precise title, but this is an opening sentence that clearly rewords the title of the essay question making it clear to the marker that this is an essay on target and focused on what the question was about. It will be important to pick out a number of examples that prove the thesis statement, such as the grandmother's choice of clothes when she gets in the car:
Her collar and cuffs were white organdy trimmed with lace and at her neckline she had pinned a purple spray of cloth violets containing a sachet. In case of an accident, anyone seeing her dead on the highway would know at once that she was a lady.
When this comes towards the beginning of the story, it is clear that this is ridiculous and humorous, revealing the pretensions of the grandmother and the way that she is more worried about appearances than reality. Note the detail that is supplied that emphasises the amount of care and thought that the grandmother had put in to her choice of attire. Of course, it is only on re-reading that this is identified as an example of foreshadowing that points towards the tragic violence at the end of the story.