I've gotten a few questions recently on how one becomes a geropsychologist. While there are many paths leading to a career in geropsychology, here is one of the most efficient routes:
1) Get a bachelor's degree in Psychology: if not Psychology, another social science (e.g., Sociology) or physical science (e.g., Biology) is also helpful, as long as you take enough Psychology classes to get a basic, broad understanding of the field.