I don't believe you have posted enough information for us to be of much help.
How old are you, and how long has it been since you have had contact with your father?
Is your mother still alive?
Is your brother older or younger and does he have any recollections you don't?
Do you have any other, older, family members that might be able to help?
When was the letter written, and what information was there in/on it?
What means have you tried? Sometimes just Googling his name ans expected location will get a hit.
Is the information you seek private, sensitive? Have you thought about posting what you know about your father so that others can search for you?
Fill us in and we'll see if we can help!