What is needed to apply to Medical School?

As long as you complete the required courses to be considered for medical school, you can major in anything. Most students will major in biology, chemistry, or something similar so they can start getting an understanding of the field and start taking upper level biology courses but I have also seen students major in history, English, etc. so they are more well rounded or have other interests.
Typically medical schools require a year of chemistry, biology, and English plus courses in physics, organic chemistry, math, and biochemistry.
In order to be the best applicant you can be, you will want to volunteer in the field - whether it is a hospital, private practice, clinic, etc. This will allow you to understand a day-in-the-life of the professionals, determine what kind of medicine you are most interested in, and speak with the doctors about their own academic and professional journeys.

This answer is for students in North America. Students outside of North America usually apply to a combination college and medical school program from high school and the total program is usually about 6 to 7 years, however the following reference may be helpful to you.

You don't really transfer to medical school in North America including the United States and Canada. First, you graduate from college with a bachelor's degree, usually 4 years.. During college you apply to medical school which is 4 years. Years ago you used to be able to finish 2 years of medical school and then attend medical school, but this almost never happens anymore. There are a few programs where you apply from high school and are accepted to both college and medical school and attend one after the other for 7 years. However, I think one should attend 4 years of college because you will be able to obtain a broader education including art, music, and liberal arts courses.

In order to apply for medical school, you need to take some very specific courses in college, usually at least one year of general chemistry, one year of organic chemistry, one year of physics, and one year of biology. Because those requirements are very specific, most people applying to medical school major in science such as biology or chemistry, but you can major in anything you want as long as you meet the medical school requirements. I have seen people major in engineering, art, classics and music and many others, so you are not required to major in science.

There are resources available to you at the Association of American Medical Colleges at https://students-residents.aamc.org, Wikipedia at https://en.wikipedia.org/wiki/Medical_school and at the American Medical Association at https://www.ama-assn.org/education, but I think the first reference is the best.