It says: Write a five paragraph essay, with proper introduction and conclusion showing how the theme of Peter Pan is developed through characters...

The best way to get started with this prompt is to identify the theme you want to explore and then the specific three characters and/or character relationships you believe support your interpretation of the theme.

From there, the simplest organization of your essay is fairly straight-forward:

Paragraph 1 -- Introduction

Paragraph 2 -- Character (or character relationship) A

Paragraph 3 -- Character (or character relationship) B

Paragraph 4 -- Character (or character relationship) C

Paragraph 5 -- Conclusion

In paragraphs two through four, you will describe how this theme is developed because of this character (or relationship) using specific examples (and I assume quotations) from the book.

My advice would be to begin writing these paragraphs first, and once they are drafted, come back to draft your introduction and conclusion.