You need an undergraduate degree before you can attend medical school. It is not important to major in a medical or scientific field; some medical schools actually prefer applicants with non-medical academic backgrounds. Medical schools prefer a grade-point average above 3.5. Know your undergraduate school's reputation, because if it is known for grade inflation the medical school's admissions office may want an even higher grade-point average.
Your medical school of choice may not require a medical-related undergraduate degree, but they still want you to have a good background in the sciences before they will admit you. Medical schools will expect you you to have taken one year of English, biology, physics, calculus and general and organic chemistry.
The Medical College Admissions Test is a requirement for most medical programs. Commonly referred to as the MCAT, it is a standardized test that helps admissions offices evaluate you against other applicants. The MCAT is comprised of four sections that will test you verbal reasoning ability, knowledge of physical sciences, writing ability and knowledge of biological sciences. Study guides and test preparation programs are available to help you prepare for the MCAT. Many schools require an MCAT score from within the past three years.
You need to submit letters of recommendation to your medical school of choice. These recommendations should come from your undergraduate professors and advisers and other people who work in health care who know you. Get recommendations from people who can give the admissions office insight into your personality and motivation, rather than from someone who can only verify that you earned good grades or were a hard worker.