Its hard to write this than the book
You're not kidding! Trick one is to unlearn (temporarily) all the great show-don't-tell scenes you put into the novel. Trick two is to unlearn most of the academic stuff you learned in school about writing a plot summary (because it's boring).
Here's the rule of thumb I've developed:
Open with a sentence that encapsulates the story in one enticing line - you'll be reusing this one, or something like it, whenever you need to supply a one-sentence summary of what your novel is about. The setting goes here, and this is also where you'll mention that it's a YA novel.
Tell the most pertinent traits of your main character - what one or two qualities will make your readers love her/him, what endearing flaw does s/he have to overcome, what makes her/him a little different and interesting, what difficult situation is s/he in? (Susan is a warm-hearted twelve-year-old whose impulsive nature gets her in trouble when she invites a street urchin to come and live with her family.)
Tell the most pertinent traits of your main character's antagonist (this could be the villain of the story, or it could be someone likeable who represents the most central conflict for your character, such as a loved family member who nevertheless stands in the way of your character getting what s/he most wants), and tell what the central conflict is between the main character and the antagonist. (Puck is small for his age with laughing eyes and plenty of charm, but when Susan's mom discovers he has sold some of her jewelry to a pawn shop, Susan realizes she has badly misjudged him.)
Unless you have written a mystery and the agent specifically says not to reveal the ending, tell how the conflict is resolved and the character grows, wrapping up the ending.
In order to make the story understandable, you may need to say something about one or more of the other characters, but keep it to a minimum.
Say something about what market you're aiming at. Don't praise your book directly (save the scintillating and magnificent adjectives for describing your characters, which will indirectly suggest that your book is also scintillating and magnificent), but do say what type of readers will most enjoy it. For example, girls between 12 and 15 who enjoy a romantic story with a Christian message, or struggling readers who will enjoy a boys' adventure story set in the Old West. If your book is similar to another fairly recent book that was successful, mention that, too: "... a quirky tale in the tradition of Lemony Snicket."
Nowadays, most agents and editors seem to want a short synopsis - one page is probably safer than two, unless the length is specified on the agent's or editor's website.