The best way to learn is by doing. Give a try at writing this letter first, and then we can help you with specific problems. [It is very difficult for anyone here to attempt to write a letter that would be appropriate to your specific situation without knowing something about it.]
Sarjay: Pretend you will be meeting the person you are writing to in person. What would you tell him? What would you ask him? Write down what you would say. You can post it for help.