Depends on what you're trying to write, but in general they're looking for an essay that really shows off your character- who you are, what's important to you, and what experiences have shaped you, be it through whichever prompt. Brainstorm, and write down who you think you are to begin with. You can even start your brainstorm just by writing down the words that you feel best describe you. Maybe pick out a memorable experience that changed you. If you're really stuck, start at the beginning, ask your parents how you were as a child, what interested you, and follow that path forward and trace your development with meaningful experiences.