Writing A Letter

Hello everyone. Sorry if this is asked in the wrong place, but oh well. Me and my girlfriend met at college a few months ago, and currently live about 1000 miles away from each other. She wants to visit me, but we are only 18. So I want to write a letter to her parents to address concerns and decent parents ought to have. I am having difficulty being both friendly and serious at the same time, especially since we have never met, and some of the content is a little awkward anyways. Any tips or pointers would be fabulous.