A typical premed course load includes at least two semesters of general biology, including a lab. After you comple your general requirements, it is wise to enroll in organic biology, cell biology, human biology or other courses designed to hone your knowledge of basic biological principles.
Two semesters each of general chemistry and organic chemistry are both typical college prerequisites for medical school. These classes will give you a solid foundation in chemistry that will pave the way for learning about chemical systems within the body as well as pharmacology, the science of medicine.
Though not necessarily required for a premed major or for admission to medical school, there are several other courses that will probably help you to develop skills you are likely to draw upon as an ER doctor. For example, psychology courses will help you better understand your patients' concerns, behaviors and attitudes. A course in addiction management and intervention will help you begin to comprehend some of the social problems you are likely to see in the emergency room. Even something as seemingly unrelated as public speaking can be a wise course choice. If you become an ER doctor, you will often be talking wtih distraught patients and their family members, and good communication skills will help.
As you make your way through your college courses, there are extracurricular activities you might wish to pursue that will add to your qualifications to become an ER doctor as well. Learn CPR and basic first aid; then consider becoming an EMT or paramedic. Or volunteer at your local hospital. Working with children in a day care setting or after-school center is a good idea as well. Or volunteer at a soup kitchen or addiction center, where you will develop skills at working with a diverse population.