I have met one person in my life who knew exactly what he wanted to do when he went to college AND he did that thing.
My cousin Don: he wanted to be a doctor when he was five, he timed my mom's contractions on the day I was born (he was fifteen), he went to medical school. He is a doctor. Weirdo.
One person--out of how many that I've known in my life?
So, if your teenager is not sure of what they want to study in college, they have lots and lots of company.
When I talk with students, I interview them about the projects and life experiences that they have valued the most in their lives. Based on my interviews, we start to pull out threads of interest that will guide them through the college admissions process and beyond.
One of my clients told me about being a camp counselor and how he enjoyed getting to know his young campers and being outdoors. He is also interested in journalism, history, travel and photography.
There are many possible life threads here: journalism, teaching, outdoor education, the travel and hospitality industry, natural resource management, working for the department of natural resources or the National Park Service.
The classes my client takes his freshman year of college will clarify things for him, a least a little bit. The organizations he joins in college will clarify things more. The summer jobs, internships and study abroad experiences he has will clarify things even more.
Don't forget that figuring out what you DON'T like to do is as helpful as figuring out what you DO like to do. My client worked in a restaurant. He checked off the food industry as a possible career path.
I have a friend who worked on a salmon fishing boat in Alaska during the summer while he was in college. He became an English teacher. I have another friend who did the same thing. He became a salmon fisherman in Alaska. He has his own boat now.
Follow your life thread. It will lead you where you need to go.