Katia, it's wonderful that you are thinking about getting an internship as a high school student.
There are a number of sites that you can use to search for internships. In addition, if you have a local clinic or hospital, contact them to see if they offer internships. Sometimes organizations don't list internships on their websites, so it is always worth asking. Contact their Human Resources departments, explain your career interests, and the type of internship or volunteer activity that you would like. Also, mention any experience that you have, such as science classes that you have taken.
Here are sites that are great for internship searches: