Hey, (redacted)! Super excited to hear you are starting the query process!
With few exceptions, your query letter will be one page, starting with a paragraph explaining why you are submitting this particular project to this particular agent -- it is typical to have done some research and be able to discuss why you think, based on other books they have represented, this book might interest them and fit their client list.
This should go something like, Dear (Agent), I am writing to present for your consideration my 85,000 word YA manuscript (title). I really enjoy the work of (one or two authors on his or her list), and I feel that (title) might be a good fit for your company because it deals with the same (age group, genre, snarky approach to the universe with an absurdist edge).
In other words, drawing comparisons and parallels to other works the agent has sold.
This is typically followed by a very high level synopsis that relates the plot of your book. Not the theme -- just the plot, though you can imply the themes through this very short synopsis. An agent or editor at this point is less interested in theme -- they will get that when they read the manuscript, assuming your plot concept interests them enough to request the full and read the story.
You want enough of the plot that the agent has an idea of the entire story. This isn't the time to be coy and withhold the ending. But also this is like an expanded version of your pitch -- what you do in your logline in two lines you can do here in 5-6.
Resist the urge to use marketing copy -- don't say things like -- This very exciting story will keep you on the edge of your seat, rollercoaster ride of the heart, etc. It is not yet the time for that language.
For instance: A lonely girl who longs for adventure finds herself ripped from her farm life in Kansas and thrust into an amazing fantasy land filled with witches, magic and Munchkins. But underneath the beautiful veneer lurk many dangers for Dorothy, including a wicked witch who wants her newly magical shoes -- and wants her dead as well. Through a series of adventures, Dorothy acquires the allies and the tools she will need to confront the wicked witch, , change the destiny of a nation, and maybe even get herself back to Kansas.
Finally, you can sum up with a bio on you -- your writing credentials, your background, platform -- selling YOU as the writing or this particular story and making a case for being a good potential working partner for the agent.
Close with your contact info.
Happy query writing!