A pitch is the answer to the question, of your book, what is it about?
1) First try just answering this. Set a timer for three minutes and write down every answer you can think of to this question: What is it about?
2) Now underline any phrase or verbs or any part of what you wrote that seems to capture the essence of your story.
Put that aside for now and try another exercise:
3) Answer this question: Who is doing what to whom? Give this another three timed minutes for brainstorming and then work out a sentence from that material.
So this might be: A woman must find her fiancé’s daughter, who disappeared while in her care. (Michelle Richmond’s A Year of Fog).
This might be: A man forced to entertain his wife’s blind friend confronts his own blindness. (Raymond Carver’s “Cathedral”)
One subject. Strong verbs. Short and sweet (and a bit deadly).
4) Notice that each of the examples also contains an inherent problem and a task for the protagonist. Try stating the central problem and the central solution the character attempts.
5) Add in a few juicy details–setting or a significant object that pertains to the plot.
A fabulous place to get more information about writing queries is Agent Query’s great write up on How to Write a Query (because you’ll need a pitch to find an agent, too).