To unlock the Take From the Rich trophy in Red Dead Redemption 2, you need rob or loot a combined total of $250 from civilians and enemies.
To loot a dead body, simply walk towards them and hold the Triangle button. The location of dead bodies will be highlighted on your mini-map with an X. When looting a body, you will pick up money or supplies. This should come naturally throughout the course of the game if you make sure to loot as much as possible.
To rob from a stranger, walk up to them and hold the L2 button to lock on to them and then hold Triangle to rob them.