How do you narrow down your interests in order to determine which field to pursue?
You should check with your guidance counselor about taking an aptitude test to see what would be your best fit in term of future career. It won't be 100% correct but it should help you decide what career you should go into.
I would recommend mapping out what you are interested in "i.e. your passions" and then what strengths you have "i.e. what do you do better than most people that you have a natural knack for." Then, I would look at the intersections of your passions and your strengths and see what potential jobs exist within that space.
Once you have a feel for that, start doing informational interviews. Find people to network with who are in that field doing that job (the one you may potentially be interested in based on your passions and strengths" and offer to take them out for a cup of coffee to ask them about their career path and how they got to where they are. They will tell you their story and through that be able to paint a picture of what their job is like. This can help you decide if that potential job could be something that you want to pursue or not. Informational interviewing is a great way to start narrowing down those interests.
Also, sometimes you just have to narrow it down by trying it out. Early in my career, I was super lost and I had a general direction of where I wanted to go but I wasn't sure which specific job to pursue. I narrowed it down to an industry (through informational interviewing) and then started trying different jobs. I mostly learned what I absolutely hated doing which I think is just as valuable as learning what you love. And along the way after trying several things, I found my passion. So, sometimes, you can only narrow it down by actually going out and doing it!