A few months ago I decided that I wanted to try out for the Zebulon contest through the Pikes Peak Writing Convention in Colorado. The goal is to submit 2,500 words of your story, write a mock query letter, and create a synopsis. Up until that point, I hadn't tried to write an official query letter. I had made a draft of one when I was a student at the Denver Publishing Institute, but that was more a trial and error attempt. It was very, very real for the Zebulon. They even created a mock agent that you had to address. I don't claim to be a perfect query letter writer, but after that experience, I do have some tips I would like to offer to those of you who are trying to get your novels published. You can try to go through a publishing company without an agent, but from what I've read, you'll have a better shot if you have an agent at your back. So, here are just a few tips:

Research your agent: Know what he/she is looking for. You don't want to send a fantasy query to a person who only accepts non-fiction stories. Look at some of the stories he/she has already chosen. That might help you decide if you have the right fit.
Include information about the agent in your query. This makes the letter more personal and lets the agent know that you've taken the time to research her. This may include mentioning the books she's acquired, or the types of things she likes to read.
Understand the query guidelines for your agent. One mishap can cause your letter to get thrown in the garbage.
Make your query letter only a page long, or follow the word count guidelines on the agency site.
Be confident, but not cocky. Make the agent believe that you have confidence in yourself, but don't be arrogant.
Be professional.
Sell your book. Create a strong attention getter that makes the agent want to keep reading your query letter. Depending on what resource you go through, you might include the hook at the beginning of the letter, or right when you discuss your story.
Don't talk too much about yourself. If you've had work published, then include that, and the numbers too of how many books were sold. If you're a beginning writer...don't say it. Just show that you're confident in your book.
Include word count in your query letter. Agents can often tell just how much revision you might need by the amount of words in your story (i.e. 300,000 words might be a red flag for a first time fantasy book).
Know your facts. If your book falls under a very popular genre that's sold millions of books, say it. This means that your book might be easier to sell, and therefore the agent might be more inclined to look it over.
If the agent asks for money upfront, RUN AWAY. This is not a legitimate agent. An agent should not be paid until your book has sold, and she'll take commission from that.
Spell check. I can't emphasize this enough. One misspelling is a good way to get your query letter thrown out.

These are just a few things that I learned. If you want additional guidance, you can check out How to Write a Query Letter.

There are a lot of resources on the internet, but your best bet is to go through the agency website to see what they require. Good luck!