How to Write a Good Query Letter

Use a simple 3-step letter (first paragraph about agent, second paragraph about book, third paragraph about you).
Be professional. Use good grammar. Keep typos to a minimum.
Don’t sound crazy (I always think this is the biggest reason that queries get auto-rejects).
Actually read the guidelines on your desired agent’s website.
Be concise. If you don’t have any credits, just say you’re a debut. There is nothing wrong with being a debut. If you haven’t met the agent personally, don’t worry about it. Keep description of book to 3 good, workable sentences.
Show that you know something about the market and that you are a reader. This means don’t mention the most obvious possible books as comparisons.
Don’t send out queries until you have finished your actual manuscript and have been through at least one round of revision.
Don’t overthink it. This does not have to be the most amazing letter ever written. The goal of a query letter is to get someone to read your pages.
If you get no requests, consider revising letter.
If you still get no requests, consider revising ms (particulary first chapter). Or writing a new one.