The Lexington militia, led by John Parker, formed up on the village green - probably more in a show of resistance than because they were expecting a real battle. The militia was badly outnumbered - about 70 of them to 700 British soldiers. John Pitcairn ordered the rebels to disperse, and they might have done so... except somebody fired a shot. Then, everyone started firing - those that weren't running, that is.
Several militia members were killed, the rest retreated. The British moved on toward Concord - though they would be harassed by the Lexington militia again on their return trip toward Boston.
Lexington's town motto is now "What a Glorious Morning for America" - though it's possible it didn't seem like on at the time. "Firing Guns and Then Running Away, Since 1775!" might have been a nice alternative. Every town needs a bumper sticker.