This document from the National Archives describes the events of April 19, 1775 at Concord Bridge as testified by men of Lexington, MA before the Continental Congress. The men, including Abraham Garfield, testified that it was the British who first fired the “Shot heard around the world” and lead to the fighting that day and to the start of the armed rebellion of the Colonist against Great Britain.
For more information on the Battles of Lexington and Concord and the start of the American Revolution you can check out the following links:
Wikipedia: Battles of Lexington and Concord
Minuteman National Park: A Revolution begins – A Nation is born
Library of Congress: Today in History – April 19