Because it's the key to getting in the front door, writers obsess over the query. This is particularly true of those of us new to the world of agents and editors. And there's more than enough advice out there -- just Google "query letter" if you don't believe me.
There are several ways of handling this. You could read through Jessica Faust's "must-read" posts or subject yourself to Janet Reid's Query Shark. You can go back in time and read the archives of the inimitable and aptly named Miss Snark.
My advice? Skip all that, go to Nathan Bransford's blog, read everything he has to say about queries, look at the exemplars he has, choose one as your template and copy the format slavishly. That's what I did.
Oh, and don't open with a dependent clause. I'd sent out 18 e-queries before I realized that's exactly what I had done.