The Minutemen won't be short of quests for you if you continue to support them. This quest will require a bit of trekking, and you can accept this mission by talking to Preston Garvey shortly after he and his crew arrive at Sanctuary Hills.
He will ask you to visit Tenpines Bluff, a modest settlement in the east, past the USAF Satellite Station. Once there, speak to the Settlers, who will ask you to clear out the Corvega Assembly Plant south of Lexington. Given the size of Lexington and all points in between, prepare for an eventful journey, including potential encounters with giant irradiated creatures and an exploratory visit in the Super Duper Mart.
Expect a lot of Raider combat inside and out of the assembly plant. The interior can be especially dicey given the narrow pathways, limited points of cover and auto-turrets. After clearing out the plant, return to Tenpines Bluff to report your success. Complete the quest with a conversation with Preston back in Sanctuary. This is also when Fast Travel begins to really come in handy.