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.