That may sound like a very ambitious career and the path ahead may seem intimidating so let's break this down.
You can become a neurosurgeon only after graduating med school. First, you need to get into med school to achieve that which requires a successful college career doing premed.
Pursuing a premed program in college can reward you with very rewarding bachelor degrees. There are many different bachelor degrees you can obtain that would still fall under a premed program. The point of these premed programs is to teach you most of the topics that will most likely come up on the med school entry exams. You DO NOT need to go through a pre med program to get into medschool, but you do need to do very well on this test. A chemical engineering classmate of mine studied for the test after work hours, took it, and has quit and has begun medschool.
Will you still want to be a neurosurgeon by time you're 20? Probably not - many things change and you may become tired of school. Medical school can take many more years and residence is a low paying grueling work period before you can make the big dollars. You may not be 30 until you make some real money, so you may reassess your career path around 20/21 when are you preparing for graduation.
You can do whatever you want in life as long as you work tirelessly at the challenges in the here and now (doing good in HS, choosing and excelling in college, etc).
In the US, to apply to medical school, you need a bachelor's degree. Any 4-year university should suffice.
Pick a college that suits your personality and a major that interests you. You will need to get good grades in college in order to apply for medical school. At the medical school I attended, the average GPA is reported to be 3.85, so even one or two B's can hurt your chances of acceptance.
Aside from this, any major is acceptable as long as you complete the prerequisite courses.
Typical medical school prerequisites include:
Biology: Lecture – 4 semesters; Lab – 1 semester
General Chemistry: Lecture – 2 semesters; Lab – 1 semester
Organic Chemistry: Lecture – 2 semesters; Lab – 1 semester
Biochemistry: Lecture – 1 semester
General Physics: Lecture – 2 semesters; Lab – 1 semester
Math: Statistics – 1 semester
English: Rhetoric (Composition) and Literature – 2 semesters
Try to find opportunities to pursue research.
Volunteer at your local hospital or low-income clinic. Ask physicians, PAs or other clinical providers if you can shadow them.
During college study for and complete the MCAT. Devote an entire summer to studying for the MCAT and consider paying for a prep course if you can afford it.
Apply to medical schools during your last year of college.
Medical school takes 4 years to complete.
After medical school neurosurgeons complete a 6 year residency for additional training. These are sometimes followed by an additional year or two of fellowship subspecialty training.