In order to obtain this Trophy your character needs to gain enough experience to progress to Level 5.
Completing quests is a good way of gaining Exp, significantly the repeating Minutemen Quests set by Preston Garvey. If you travel to Sanctuary Hills after he and the other settlers move in, Preston will continually issue quests to set up new settlements and defend old ones. Each quests takes between 10-15 minutes and will award you a generous amount of Exp, taking you to Level 5 reasonably quickly.