To be transparent, I did not even know what I wanted to do when I graduated college. I had lined up a career before I graduated in a Customer Success role, which was quite entry level, but my point here is that it's ok to not know right away. Sometimes it's all about trial and error and you shouldn't feel pressured to know immediately. Within my time in CS, I uncovered that I had a passion for sales and driving revenue for the company- which led me to my current role.
Whether you discover your passion in high school, college or after college, everything will fall into place!
You've received some great advice to your question. I would just add that although there are some people who know exactly what they want to do early in life or when they graduate from high school, this is rare. Some have interests that they would like to pursue and along the way in college it becomes more clear where they would like to go with their career, some even find their dream career in college taking a class or two and finding something they love and want to do the rest of their life. Many of us found careers doing what we love and we didn't know that career even existed when we were in high school. I did know that I really liked business, I just didn't know exactly what I wanted to do in business, once I was on the inside, I found my dream career. I agree with Doug that an internship or temp job with a company of interest to you is a great way to sample different roles, see things on the inside, maybe see a career you didn't even know existed, or define for you the direction you want to head with your studies and career.
Hope all goes well for you
Sometimes it helps to expose yourself to new things to have a better idea of what's out there or even spark an interest. Or maybe exploring other types of classes in college might reinforce the idea that your initial interests are what you want to continue to pursue.
I don't think there's a right or wrong answer when selecting your career and your interests. You should write down a list of your goals and interests. Once you have put some thought onto a paper, you can start researching the different fields within STEM that matches them. I found that it was important to talk to friends and mentors on your career path. They could give you excellent guidance that you would not have gotten by Googling on the internet.
It's been 4 years since I graduated from Davis, but while I was in school I switched to Economics and International Relations instead. You don't have to have your career figured out. The best thing I can recommend is, take a variety of classes your first year in college. If you want to be pre-med take a few general pre-med classes, but take classes for other subjects you may find interesting (economics/business, english, etc). What's great about careers is you can use a variety of paths to get to the same end goal.
If you for sure want to do STEM look into schools with a variety of programs so you have options to try different things on top of your STEM classes! I'm in a stem field (Tech) but not a stem major, so there are a bunch of options!
That led me to another start up doing international financial reporting... More databases, more coding... Then I got tired of start ups.
My technology led me to an opening with MCI (Verizon today) doing Project Management in the IT organization. This became the first part of what I would consider my true career. Within Verizon, there came a point where it was time to shift what I did or go find work elsewhere... My career evolved again into Business Process Engineering (Lean Six Sigma, Design Thinking, etc...) and that led me to other aspects of my career.
The moral of the story is that your view on what you want your career to be will evolve as your life evolves and opportunities become available. The thought of working in a think tank, writing papers analyzing the latest coming out of Russia is still fascinating to me, but when the time came to make some decisions on what was important in my career, that was not as important as some other factors.
If you want to follow your dream career from an early age, make sure you live a life which that dream can afford. As they say... if you love your work, it will really never feel like work!
Hope the story helped. Best of luck to you.