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As a result of King Phillip's War, by law all men in Massachusetts were organized into militia companies. A select group of the militia were minutemen, ready to respond in a minute. The first shots fired at the Red Coats by the militia were fired by the Acton minutemen at the Old North Bridge. Isaac Davis was the captain Acton minutemen. When asked if he would lead the first attack on the King's men, he replied, "I have not a man who is afraid to go."

Davis was a gunsmith and had armed his company with bayonets and cartridge boxes, instead of a powder horn, so they could fire at the same rate as the regulars. The Acton company was considered the best drilled and best equipped.

From historynet

Seeing the colonials coming, the British retreated over the bridge. The last men across began to tear up planks in order to stop the advancing force in its tracks. Major John Buttrick, the British commander, called out, ordering the colonists to halt. His soldiers, meanwhile, assumed battle formation. When the colonists neared the bridge, the redcoats fired a random volley that wounded fifer Luther Blanchard and Jonas Brown of Concord.
The next British volley fell short, but served as proof that they meant to fight. As the colonists prepared to fire their muskets, the British fired again. Davis, just then raising his gun at the king’s men, fell dead, shot through the heart. A private in his company, Abner Hosmer, received a mortal bullet wound in his head.

Bruiser 10: "From what I can tell, the Minutemen were armed with military-type weapons."

They were, ahem, "a well regulated militia" -- regulated by the Massachusetts Provincial Congress -- who stored militia weapons & ammunition in locked buildings in small towns like Concord.
But in February 1775 the British Parliament declared them to be in rebellion -- effectively a declaration of war -- and British forces began moving to suppress the rebellion.
Lexington & Concord were the first major clashes of American militia against British regulars.