To ready yourself for any degree it is important to prepare well before college. As early as your freshman year in high school begin earning as high a GPA as possible; remember you are going to be competing with many other applicants for entrance into the college or university of your choice and for possible grants and scholarships.
While in high school you can do much to make your college years easier. You will be doing endless amounts of writing, studying and the like. Learn to keyboard efficiently, learn to write well and load up with all the classes you can in social studies -- after all you are preparing to learn about the mind, so begin learning what makes people tick.
There is a long list of classes that must be taken in order to earn your degree in psychology. You can usually get a syllabus by checking out the school or schools of your choice online. You will notice that they vary slightly from one institution to another. However the core classes are pretty standard. They include Research Methods, Cognitive Psych, Clinical Neurosciences, Learning and Behavior, Sensation and Perception, Theories of Psych, Social Psych and many others.
In the state of California, for example, you must have a degree in psychology or in education to be eligible to become a psychologist. You must also take state licensing exams and pass in order to legally call yourself a psychologist. State requirements vary, however, so make sure to check requirements in the state or country where you intend to practice.