The first raid is handed to you during the main storyline.
During the quest "Settling Down", you'll talk to the only Blacksmith you have that'll have found a map during your first explorations of land in England.
Simply follow the questline to unlock the trophy.
During the raid there will be two chests to raid which will complete the raid. The two buildings are on fire, so you can't miss them, if you are struggling, our lovely Raven friend can just about locate them through the smoke.