For those students who start thinking about MCAT ahead of time, GREAT! With careful planning and knowledge, you will make your MCAT journey much easier.
MCAT tests on the following content: biology I & II, biochemistry I, general chemistry I & II, organic chemistry I & II, physics I & II, psychology and sociology. Even though psychology and sociology are not required, taking them as courses will help you tremendously for the Psychological, Social, and Biological Foundations of Behavior section of the test.
To see which topics you need to know for sure, check AAMC MCAT guide https://students-residents.aamc.org/applying-medical-school/article/whats-mcat-exam/