The medical field is a great choice. As a high schooler it would best to further refine your study skills and time management skills. Getting a hold on these two even before going to college will help greatly. Another thing to consider is how well of a note taker are you? Do you have system to prepare for a test, quizzes and text reading? Flash cards, information scanning and pre-reading chapters are a good way to lay a foundation to be prepared academic wise.
It seems as if you are considering a career in the medical field. A good determiner for success would be to full evaluate your performance in all of your math and science coursework as those subjects are the bedrock of any medical specialization. Then you should also think about what path in the medical field you wish to go down (i.e. Medical Doctor, Dentist, Nursing. Physical Therapy, etc). You would want to also consider what colleges/universities have a good pre-med undergraduate program as well.
Best of luck to you.