When writing letters to get to know a person, what do you write about that won't seem so self centered?

Ok, so I'm currently "talking" to this guy I have never officially met before while he is finishing up technical training for the Air Force in Texas. After talking on the phone a couple of times we realized how much we had in common. Now that school's ending soon and he's got about a month and a half left he doesn't have too much time to shoot the breeze on the phone. So we've started letter writing but I'm having trouble.. I want him to know more about me, my personality, my life, my plans.. that sort of thing.. BUT I don't want to sound self centered, conceeded, or anything else that would make him think those things.. We meet in a month and we have so much in common! I just don't know where to start in a letter..

See additional info for the background of the situation - thanks in advance! : ]