Hi Matthew,
Welcome to our forum!
A letter is a personal item and should reflect what
you want to write to the celebrity. However, here's just a rough idea of how your letter format can look like:
Introduction

"Dear ________"

"My name is _____ and I am a ___-year old fan of your's from (city), (country). I'm writing to tell you how much I admire you, and to kindly ask if you could please sign an autograph for me."
Body

(here, you can write about yourself, how you became a fan of them, what you like best about them, etc.)
Conclusion

(in your conclusion, you may re-state your request for an autograph)

"Thank you very much for taking the time to read my letter. I wish you all the best in the future for both your career and personal life"

(sign off with your name)
Hope this helps!
