You've grabbed your editor's attention with a great opening and outlined your take on an area of interest for her readers. In the closing you must sell yourself. This is the part of the letter that is cousin to a resume. Tell what qualifies you to write the story whether it's life experience or writing clips. If you're a beginner, do not say you have not been published. Instead, describe your experience or research on the topic.
This is the place for experienced writers to say where you've been published and include clips with your letter. If you submit your query by e-mail, be sure to include links to your clips and/or your website.
Don't miss the opportunity to ask for the assignment. I usually end my queries with something like, "I appreciate your consideration of my story "Title," and I look forward to hearing from you. All that is left is a polite closure. I like to use "Blessings" for the Christian market and "Kind regards" for the general markets. Be sure to include your contact information in your signature.
If you haven't already done so, I hope you'll purchase your copy of Queries and Submissions by Thomas Clark.