I think I would select to write to someone I respected or had questions for. Here's an example of a letter I might write to a celebrity:
Dear Angelina Jolie:
I was to write to you to express my sincere appreciation for what you've done in the public eye over the last few years. The women in my family have suffered from a history of breast cancer, so it is inspiring to see someone speak about these issues in the media today.
I think you have immense courage for sharing your story and choosing to be proactive about your health. I understand your desire to be around for your children in the years to come.
I hope that you realize what an impact you have made on those around you whether you know it or not. I wish you all the luck and health in the world, and I hope we continue to hear about your triumphs in the year to come.