To become a Neurosurgeon, you first need to finish medical school--so you will need a bachelor's degree (undergraduate) which takes about 4 years, then to finish medical school which is another 4 years. Some schools have combined programs, which can be completed in 6 years.
After medical school, you will need to complete a residency program in General Surgery, which varies about 3-4 years. After that, you would need to complete a Fellowship program in Neurosurgery, which is approximately 7 years long. I know that sounds like a long time, but the training needed for a neurosurgeon is intensive because... well... you are working on people's brains.
So, the total after high school is approximately 18 years long.
Best of luck!