Have you heard of the "3 paragraph" rule for queries? You know, where you're supposed to tell everything that's important about your manuscript and you in just 3 paragraphs? That's a laugh, isn't it !
I'm going to talk about what I've been taught about this rule. You might find it handy, or you might not. Anyway, supposedly the first paragraph is actually no more than a one or two sentence introduction. This is to try to make the reason why you're querying this particular agent a personal one, i.e. to let him know that you've done your homework and know something about him. For example, if you can start this first paragraph off by saying...I am seeking your representation because you represented TITLE OF BOOK by AUTHOR'S NAME, and I think my manuscript is similar. However, before you say this, you need to make sure you've read this book and your book actually IS similar. Nothing will put an agent off more than to ask for your manuscript based on this kind of statement in your query, and then find that your story doesn't even faintly resemble the one you mentioned.
The second paragraph should be a short and concise description of your plot/storyline. By short I mean only a few sentences, maybe 4 or 5. You don't want to try to work in subplots or characters' names. Although, not mentioning even the MC's name seems to be something that agents disagree on, because I've seen some agents who want to know right away who they will be reading about. So there seems to be some disagreement on that. What you do need is tight and concise writing: instead of "My novel is about a tragedy that happened at a high school," you should say "My novel is about the 12 students who were shot at XXX high school." The second thing you need is the location of your story. Not "It takes place in a large metropolitan city," but "It takes place in Chicago, Illinois." And the third thing is the time period.
If you are abiding by the "write tight and specific" rule, then you want your time period to be tight and specific too. Instead of saying "My novel takes place over a short span of time" you can say "My novel takes place during the month of August," or "over 10 days in December." This gives the agent some idea of the structure and time line of your work, and the knowledge that you've done your homework and the story isn't just something you've thrown together.
The third paragraph, which should be the final one in your query, is your biography. And this is also the one which probably causes the writer the most difficulty. People are always asking What should I put in my bio? How much is too much or not enough? Is it long enough, or short enough? Again, this paragraph needs to be short.
You don't need to, and probably shouldn't, mention "minor' credits, like being published in a local newspaper or magazine...anything that seems "amateurish" should be left out, even if you were delighted that you were published in something local. Anything major, like a children's national magazine, is a great publishing credit. Any major award, like the Newbury Honor Award, or the William C. Morris Debut Award, should of course be listed. Anything that tells the agent what accomplishments you already have behind you needs to be included. Any extra schooling related specifically to writing, such as an MFA or writing education courses, such as the Institute of Children's Literature, is a bonus.
Another couple of things not to do: Don't get personal, about how you started writing in middle school; what your hobbies are, how you write the office newsletter and everyone thinks you're great, and so on. Don't include information that simply is not relevant to your writing, just "talking to be talking." Don't embellish your bios by talking about how everyone thinks your writing is great, your daughter's 5th grade class loved your last story, and so on. Be specific, concise and relevant in all that you say.
That's a wrap. Let me know what you think.