Seriously there are hundreds of sites with actual examples that can help you. If you are like me and previously unpublished, you need to write a query that will make your lack of publishing credentials fade into the white (of the paper that is). In many cases, your query will be an e-submission. A lot of agencies are going green these days. However, that DOES NOT mean that your letter should feel like an email. It should be professional. Make sure your subject line includes the genre and states clearly what your novel is about.
Your query is your one-shot interview, your audition, your foot in the door.