1. How do I begin a query? What goes in it?
Start with the reason you're sending queries in the first place: your book. State the genre, word count and title in the first sentence. Then go on to the plot. Don't be vague here. Give us actual plot. Don't use cliches. Tell me the main character, what s/he wants and what stands in his/her way. If you've got any publishing credits, the next paragraph is a great place to put them. Don't include self-published/vanity books or poetry. If you don't have anything, thank the agent and sign off.
These are business letters and are not the place for gimmicks such as a paragraph from your book, or a query told from the point of view of your main character. Also, don't write them in third person. That's just weird. I would not include a paragraph from the book nor would I write the letter in the “voice” of one your characters—those are gimmicks.
If you have a nice and tight pitch you can jump right into that if you want. You can also try to establish a connection with the agent. Did you meet them at a conference? Say so. Were you referred to them by someone important? Say so.
However, don't include that you like to whittle in your spare time and you have four kids ages 4, 6, 7, and 9 that all play little league hockey and like to fish at the weekend. There will be plenty of time to get to know you as a person after the contract is signed. For now, all that matters is your book.
2. I'm 15, should I mention this is my query?
Usually the people that ask this question are the very young and the very old. They are generally defined by their age in their daily lives. In writing though, it's not really an issue. Agents may gawk at a younger querier thinking that they haven't had enough experience yet or haven't spent enough time writing to be good yet. On the other end, agents are looking for career writers, ones that will continue to publish books for awhile. If you're older, they might think you've only got one book left in you. Try writing several books and mention this in your query so they know you're serious and not just trying to push out a memoir in your last days. To be absolutely safe though, don't mention your age at all unless you're under 18 and about to sign the contract.
3. When should I query? How do I know if i'm ready?
There's not really a good answer to this question. If you've just written THE END, it's not ready. Spend some time revising, get a critique partner or two, and then some beta readers. They should let you know if there are any obvious errors or concerns and you can then revise again to fix these. By then, you should be really sick of your book. Put it in a drawer for a few months while you start another project.
After a while, pull it back out and give it a read. Do you still love it as much as you did when you first wrote it? If so, great! Give it another revision and you should be able to start querying. If not, give it another revision and a few more beta readers. Then revise again.
4. Should I mention that my book has been professionally edited or is copyrighted?
No. All work is copyrighted the moment you write it down. Saying that in your query or including "(c) AUTHOR NAME, 2010" in your query only comes off as amateurish. On the same note, everything that you query should be edited, so saying that the work is edited (even by a professional editor) also comes off as amateurish.
5. How many agents should I send my query to? All of them?
Not all at once, no. If you send your query to 100 agents and your book isn't ready, you've lost your chance at those agents. Send it out to a few agents, 3-5 of them, and see what comments you get back. If they all want fulls, great! If you get all form rejections, revise your query and/or your sample pages. Then send out to the next 3-5 agents and see what they say.
Make sure that you send each query separately. A personalized query addressed to a specific agent is best.
6. How should I format my query?
Times New Roman, single space, 12 pt. font. Take out any formatting that you might have from Word. Unique formatting, cutesy pictures, and designs (including email themes) do not belong in query letters. A typical query is 4-5 paragraphs with 3-4 succinct sentences each. The sample pages vary from agent to agent (READ ALL SUBMISSION GUIDELINES CAREFULLY AND FOLLOW THEM) but should always be included IN the email after the query, not as attachments. Most agencies will not open attachments, even if you said you put them there and that they aren't viruses. Some agencies delete these emails without even opening them at all.
7. Should I mention that the query is simultaneous or exclusive?
You can, but you don’t have to. If you say it’s exclusive, they understand that they will be the only ones who have it. If you say nothing at all, they will assume multiple agents must be considering it. Always check each agent’s submission guidelines; a few rare agents will specifically request to be informed if it’s a simultaneous submission or request that your query be exclusive to them.
8. Can I send a query for a short story collection?
95% of agents do not accept short story collection queries. Collections are a really hard sell unless you've already got a following. Even JK Rowling's Tales Of Beetle The Bard didn't sell nearly as well as her Potter books. If you have a collection of short stories, you can do one of three things: 1) Repurpose some/all of the stories into a novel. 2) Write a new book, a novel this time, and sell that first to establish a reader base. 3) Query the few agents who do take collections and hope for the best.
9. Can I query multiple agents at the same agency?
Generally, no. A rejection from one usually means a rejection from the entire agency. Usually, if you query one agent and they think it's not right for them but still has promise, they will pass it on to fellow agents in the office. Check the submission guidelines, sometimes they say if it's okay to query another agent after a few months have passed.
10. Can I ever re-query the same agent?
Sometimes. Don't re-query with the same book and the same query hoping that the agent might be in a better mood that day. Make sure you've either rewritten your book and query substantially or it's a query for a new book entirely.