What should a high school student focus on if they want to get into the medical field? What courses should they make sure to take?

Hard to answer without a little more information. When you say courses, do you mean college courses or do you mean high school courses? You say "high school", so I'm going to assume you're asking about classes you can take right now, and other things you can do. Here are the three things you can do that I think will be most impactful for you:
1. Take every hard science course you can take (in the rough order of biology, chemistry, organic chemistry, and then things like physics)
2. Get an internship in a hospital, doctor's office, pharmaceutical company, the United States FDA, or other medicine-related organization. By getting this internship your goal should be to learn as much as you can about the field and try to come to further clarity on what specific sub-specialty you're interested in and why. The why is important because med school is very difficult (and expensive), so you're going to need to keep your "why" top of mind as you navigate the challenges to come.
3. Look for opportunities to turn your summer into fruitful activities. Most students squander their summer doing things that aren't actually helping them reach their goals. You might, for example, find some type of program that will allow you to provide medical support services for people in need in developing nations. You might be able to find a volunteer opportunity working in a local health center or soup kitchen. When you're there, your goal should be to add as much value as possible. To find the biggest problems your superiors have, and try to solve them on top of doing your "regular" job. That will really make you stand out from the crowd, and will come in very handy when you need to ask people for recommendations for college or med school or other programs.

Good luck! (Was that helpful?)