This is really three questions, and rather than answering them all in depth I will answer them briefly and then direct you to a place where you can get more detailed answers.
Plot basically refers to the sequence of events in the story. That means what happens. In short, a boy named Jonas is chosen as the Receiver of Memory and learns some terrible things about his supposedly perfect community. Here is a link to a full summary:
Theme is the moral or message of a story. Another way to look at it is that it’s a lesson to be learned. This book is a dystopia, meaning that it seems like a perfect world but turns out to be horrific. One of the themes is that difference should be celebrated, not feared. Here are some links to themes:
The setting of a story is the special time and place of the action. In this story, we don’t know the exact time or place. The place is the community, and we don’t know where that is. We know that it’s December when the story begins, and that the story takes place over one year. But we don’t know what year it starts. Here is a link to information about the setting.