Hi dewdrop10
Here's a few rough ideas on what to say in your letter. Feel free to go about it however you wish:
1. Start off by introducing yourself
2. Then, mention that you're a fan of their work in <something> and why you like their work. Don't go into detail about the background of what they did (they've probably heard it a thousand times before) - just what and why you like their work.
3. If you want something autographed, or want a photo, politely ask for one somewhere in your letter. Don't ask right at the beginning, or ask mid way through when you're mentioning something else.
4. Thank the celebrity for their time, and wish them all the best.
Don't forget a SASE or SAE + IRC(s) when sending out your request

. It makes it much easier to have a response sent back to you.
Also, don't make your letter sound like a form letter (i.e. you've filled in the blanks to a basic template.) Make your request sound unique.
I hope this helps. If you have any further queries, please feel free to ask!
nowandagain [rockon]