Target any STEM AP courses available to you in high school, including math, bio, physics, and chem. All of those will be helpful foundations for more advanced topics in any science field. If available, take any anatomy or physiology courses you can as well. See if your local college has a high school plus program to get college course credit in your senior year, and take any topics that fall under engineering or biomed focus areas.
I went in to college interested in pre-med, with a focus on chemistry, but switched to computer engineering mid sophomore year. Having all of the AP courses going in meant that the year spent in another major didn't put me behind.