Before you're able to claim your major, you'll likely need to take a number of general education classes that are nonspecific to your future major. Math, astronomy, philosophy, English and science courses are common for the first year of university or college. They are meant to prepare you for work in your desired majors. You may have to amass all of your general education courses before you can move onto a full psychology-driven course load.
You may find that an introductory course for psychology is included in your general education credits. If not, you'll certainly need to take and pass Intro to Psych. Not only does it prepare you for the terms, course matter and work needed to complete a psych major, but it acts as an ideal chance to decide whether or not a psychology major is the right course for you. Your introduction to the program may help you become more interested in the course, but it could also help you to realize that you prefer another major.
Some universities and colleges require participation in laboratory classes in order to declare psychology as your major. This is because psychology is a research-driven major. Spending time in a lab with a life sciences class helps you to better understand and practice correct research and reporting techniques. You'll likely continue a number of laboratory and research classes, so your first life sciences experience may be indicative of how you'll do in the rest of your labs.
Every school is different in its requirements for declaring a psychology major. The number of credits needed and what classes are required will vary from school to school. Making an appointment to talk to a guidance counselor or a department head can give you a clearer picture of what is needed to enter a psych major at your particular school. Course catalogs and school websites can also be helpful resources when preparing your course load.