The first paragraph should be a brief synopsis of your novel. No more than 100 words, perhaps 150 words. This synopsis is written in the present tense. You want to hook the agent's curiosity to read more. It's a challenge to condense a 100,000 novel down to 150 words, but important you do. Think of the synopsis as a back cover blurb. It gives just enough information to create excitement about the novel. If you need some examples go to the bookstore and look at the back cover blurbs of books in the same genre as yours.
The second paragraph describes the market for your novel and includes the genre. Are you aiming for romance readers, cozy mystery readers or is your book a young adult novel? You can compare your book to other similar books, saying something like "readers of Sandra Brown will enjoy TITLE OF YOUR BOOK. Mention the word count and that the novel is completed.
The third paragraph includes your background as a writer and a brief bio. If you have writing credits make sure you mention them. Many agents don't consider a vanity/subsidy book or self-published book as a writing credit. If you include the title they will look it up on amazon.com to see who the publisher is.
The third paragraph is where you can describe any special experience you have that could become a publicity hook or gives you an insight other writers might not have. For example if you are a pastry chef and your heroine is also a pastry chef that would be relevant.
End the letter by thanking the agent for their time and consideration and asking if they would like to see the first few chapters or a completed manuscript. The purpose of the query letter is to motivate the agent to ask for the manuscript.
The query letter should be no more than one page single spaced. You could also include a 2 or 3 page synopsis of your novel, but don't include the first chapter or the entire manuscript.
A great query letter is the first step in "how to get a novel published."
