You need to understand that undergrad—> grad school doesn’t work like high school—> college.
In college you study and focus on a single discipline. If you’re a bio or chem major, you won’t have the necessary background required for grad school in psychology and won’t be accepted. You’ll need to take additional psychology courses (usually around 8-10) to meet the minimum requirements for grad school admission and you’ll have to have enough experience with the discipline to identify a sub-specialty you wish to focus on.