During mission 2 you have to get drop a air crate full of food to the ground after which you'll have your first encounter with the reapers. After the fight you will be given a choice of shooting people or standing around for a little bit to allow the people to get food.
You will want to pick the good option of waiting for others to pick their food up, you will be greeted with a tutorial on the game's karma system and your trophy should pop.