There is a huge amount of information out there on the internet about how to query, what to include in your query letter, how to find the right agent for you, and so on. I'll try to discuss some of this on later posts.
One thing I did just want to mention is the importance of following the submission guidelines for the particular agent or editor you are querying or submitting to. While most of the guidelines are fairly standard, some agents/editors do ask for different things. An example that springs to mind is the agent who wants you to include a paragraph on how you see your book being marketed. Other agents specifically ask you to advise in your letter whether or not you have submitted your query to publishers.
What you don't want to do is to leave out something in your query letter that will cause the agent/editor to press "delete" or move straight on to the next query letter. It's hard enough to stand out in the hundreds of queries an agent/editor receives each week without shooting yourself in the foot in this way.
Perhaps as importantly, read that agent/editor's personal blog. More and more industry professionals are blogging these days, and they often include information in their blogs about their own personal querying preferences. They often also tell you what they DON'T want to see in a query. This insider information will be invaluable when it comes to distinguishing your own query from the 150 others submitted on that day.
Best of luck with your queries.