What do you put in the letter? What if you have no experience, or only very limited experience. Should you write about yourself in the third or first person when explaining your experience?
There are multiple parts to a query letter. The first part is you. What kind of writing experience do you have? Have you published four books already? Do you have a dozen short stories in various magazines? Have you won any writing awards? Do you have any experience in the publishing world - editing, lit agent, etc. This is where you want to "sell" yourself. If you're writing non-fiction include what gives you the authority to write a book. Are you an emergency room nurse and you're writing about first aid tips or what live in the emergency room is like? This would be written in first person, unless the publisher specifies otherwise.
The next part is about your book. You're going to want to give them a summary about what the book's about, who the characters are, and how it ends. Yes, they want to know how it ends. Don't leave them with a "Will Bobby survive his detention-from-hell?" That's going to annoy any publishers or agents reading the query. If the book has been published before, you'll want to let them know that, where, how long ago, and if rights have reverted to you. If it's been published before you'll also want to let them know if it's won any awards.
Just like you'd want someone else to look over your novel for errors and mistakes, have someone proof read your query letter too. Nothing will sink your chances of getting published like a query filled with errors.
Different agents want different things. Some love innovative, chatty query letters. Others want "just the facts." Your best strategy is to find an agent you would like to have represent you and read their query letter guidelines.
If you're having trouble finding agents and agent information on the Internet, buy the most recent copy of Writer's Market you can afford.