Well, to find the main idea, think about what the story is mostly about. Is there a central problem or theme that everything ties back to? Also, notice repeated ideas or events, they might point to the main idea.
You can start by looking at the beginning and ending of the story. Usually, they give hints about the main idea. Also, pay attention to the characters' goals and the problems they face.
The main way to find the main idea of a story is to summarize what happens and figure out the most important point. Consider what message the author is trying to convey. Sometimes, it helps to ask yourself what you would tell someone else about the story in a few sentences.