If you are interested in becoming a US Navy Corpsman, you will need to talk to your local Navy recruiter. They can help you get in the program to join the navy and become a Corpsman.
In the mean time, make sure you are taking care of your body by getting into great shape, make sure you finish High School with good grades, and stay out of any trouble. If you have time you can take classes at a local Community College towards an EMT certification that will help you in your career.
To provide additional information to Jeremy's answer, it is important that you prepare yourself through development on the subject matter. The aptitude test will align you with your inherent capabilities. It is best to speak with a recruiter and take time to make sure you have some level of knowledge if it is your passion.
Michael recommends the following next steps: