Where to start will depend on where you currently are. If you're a high school student taking courses in organic chemistry, physics, anatomy & physiology, biochemistry, and psychology can all be beneficial while still in high school. Starting volunteering and getting involved in leadership positions is also something important that you can do as a high school student. But the pathway to medical school really starts in college. You need a bachelors degree to go do medical school, it is common for people to major in biology because the degree path encompasses the medical school prerequisite courses, but you can do any major. For example, I have a nursing degree and I have a friend who's degree is engineering. The important thing is that you get all of the required courses. Medical school requires 1 year +lab of general chemistry, 1 year +lab of organic chemistry, 1 year +lab of physics, 1 year + lab of biology, math (often statistics and calculus),1 year of English, psychology, and biochemistry. There are many more recommended courses like genetics and some schools have other required courses as well. It is important to work with a premed advisor or committee if your college has one to determine what courses you need. You will also need to maintain a high GPA and participate in a variety of extracurricular activities. Medical schools want volunteering, leadership experience, research, and shadowing of physicians. These are all time consuming things that are best to get started on sooner than later. Best of luck!