Well you do need to ask some questions so that they will be able to have something to talk about in their next letter. But try to tell them something interesting about where you live or interesting things that americans like to do, or what not. I had a japanese exchange student and I tried to make my letters interesting because I know how it is to receive a boring letter.

well you should tell a little bit about your self and what you do and then ask him questions like do you work what age do you get to work at i heard you could go partying at a young age there you know along the lines of that.
and then in the next letter he will ask you more about you about you since you told him a little about you he will have more to say and you will ask you more question you and so on you get it