Pen-pal for a marine, what to write?

I'm sitting here about to write the first letter to my pen-pal in the marines. He's twenty-one and that's basically all I know about him. What are things I can write to him about? What are touchy subjects I shouldn't ask about? I don't know if it should be a more somber letter or if I should just speak plainly. (which is usually more upbeat/quirky)