You will need to be first qualify for a medical degree, and then later specialise in neurosurgery. To get into medical school in Kenya, you should aim to pass your sciences (especially biology and chemistry), mathematics and languages. Even though these are the key subjects needed for medical school, you usually need to get As in all your subjects due to the high competition for places. Once you are in medical school, you will find more information on what is needed to specialise in neurosurgery.
I can add to the extensive answer @Jared gave. Essentially the process in university starts with about 5-6 years of a bachelor in medicine and surgery, where you do several rotations including internal medicine, paediatrics etc to expose you to different medical fields. During the last years before graduation, you will do what’s called an elective, where you get to work at a specific hospital to gain practical experience for a short period of time. After graduating, you do an internship for about a year after which if you decide to specialize in Neurosurgery you can do a Masters in Neurosurgery which takes an estimated 5-6 years or so. It’s a lengthy process, which you can start by first focusing on your studies in high school and take it step by step 🙂