1. Subject line should say "Query for TITLE"
TITLE means the title of your book. Yes, I have gotten queries from people who didn't understand that.
2. The salutation should be Dear Janet, Dear Ms. Reid, Dear Snookums; or some variation of that. It should not be To Whom It May Concern, or Dear Agent.
3. The first paragraph should be what your book is about. It helps if you start with the main character's name, then tell me what choices he must make, or why things are going to change for him soon.
There are examples, both good and bad, of how to do this at QueryShark.blogspot.com
4. Include the word count. The novel must be finished before you query.
5. Tell me which ONE category suits your novel. Here are the categories:
6. Include the first 3-5 pages of the novel in the body of the email.
The first 3-5 pages are not the preface or prologue. They are the first 3-5 pages from chapter 1.
7. Include your contact information
8. Do not attach anything.
9. Do not use any color font other than black. Do not use any background color other than white.
10. For a lot more posts on the subject of queries, read the posts labelled "Query Pitfalls" here on the blog. You can find them by looking at the very bottom of the blog roll on the right side of the blog under "Blog Posts Sorted by Category"