Excellent advice from Greg Mankiw:
Suppose that an 18-year-old student comes into a science classroom and says, “My grandmother is ill with a serious, rare, and hard-to-diagnose disease. I want to become a doctor to help figure out a cure.” What should the student study? Probably not this specific disease, at least at first. The place to start is with basic biology, chemistry, and so on. Only after these fundamentals have been mastered can the student go to medical school, become a doctor, and be in a position to study the illness that motivated him in the first place. Much the same is true with the study of economics.