Southeast of Concord and west of the Starlight Drive In is the Drumlin Diner.
You'll find the diner's owner, Trudy is in the middle of a disagreement with two Raiders over money that her drug-addicted son owes. You can trigger the quest if you agree to help her. Note that if Codsworth is with you, he won't like this decision.
Kill the two Raiders and report to Trudy to complete the quest.