What are some ways a high school student is able to answer the age-old question: "What do I want to do for the rest of my life?"

Get yourself out of the "Forever job" model. Most people will have many jobs or even whole careers in their lives. As you are in High School, and people are asking you "What do you want to do in your life," start with big generalities.

Plan right now to be a life-long learner.
Unless you are planning to be an entrepreneur (run your own businesses), or go to work in the family business, you will probably need to get at least a Bachelor's degree.

I had planned to be a business owner so I didn't put a lot of thought into college at that point. If I were to do this today, I would have planned to take a Bachelor's degree from the beginning.

I did go to work in the family business (commercial boarding kennel) as a general manager, but I still wanted a way to escape that if I changed my mind later.

So I worked on an Associate of Science degree in Watchmaking (this field still exists, but quartz watches tore the hull out from under it before I finished my degree).
I got a contract job as a watchmaker at 19. Did that for about a year, and went back to the family business for a while.
As an extension of the family business, I ran my own mobile dog-grooming business for a while.
I went to work as a watchmaker, for a while, and as a fast-food cook for a while. I have almost always worked 2 or more careers at a time.
I am a polymath, and have always been interested in finding out about new things in all sorts of areas.

You may be interested in only one thing right now, but the careers you are going to put most of your time into may not even exist at this point.
When I was in High School, there was no such thing as the Information Technology discipline, or Game Design.

Some general career interests dovetail nicely with others. For instance, my own early music interest made sense with a later computer programming interest. I know a lot of people who have Liberal Arts and Sciences Bachelor's degrees who ended up in computer security, computer programming and computer networking.

Back to choosing a path: If you had always wanted to be a medical doctor or an orchestra conductor, you would not be asking your question. There are people who have very well-defined career goals, but that was not me; and looks like it is not you either.
As a general guide, for starting your higher education, think in terms of what could I stand to focus all my time on for 3 to 5 years as a job. In any Bachelor's program, you will get a taste of a lot of other things. Think of this as a buffet of learning. Try everything, as you don't know if you will like it or need it later. There is nothing wrong with liking more than 7 topics at a time.

If your favorite thing is math, technology, and things; look into engineering, computer science, information technology, physics, and other "hard sciences" things.
If you are most interested in interacting with people; look into business management, social work, psychology.
If you gotta dance, gotta sing, gotta act, gotta play that trombone; there are programs in all of these and more performing arts.
And so on...

Hi, Karen. Everyone else has given great responses. I agree with them.

What you think you may be committed to now may not be what you ultimately decide in the future. Don't stress. Consider working at different jobs during high school and college part time. It may be best to start your college journey off with basic required courses first. Also consider getting internships every summer. This will help you determine what you do and don't like AND helps you build your resume. It also helps you build more skills and great connections. I also suggest getting "Strengths Finder 2.0." It is a great book that helps you identify your top 5 strengths. I wish you the best!

Hi Karen! My guess is 60 or 70% of the current working population haven't answered that question yet. And even many of those are not financially self reliant. I think the question suggests there is some ideal work scenario that if you pick right will create a wonderful life. That is a very misleading assumptionI. I also think there are other important things you might like to know first. Like what are my gifts, talents. interests and preferences, Read Rich Dad Poor Dad by Robert Kiyosaki (Library) and explore his perspective. I graduated from college at age 29 with a 2.4 grade point average with no clue who I was or what I wanted to do and had six jobs the first two years out of college. I finally got some clue at age 35, and changed careers paths twice after that plus being fired at age 53 in between. Relax life is a journey not a destination. Be more focused on growing more self aware, being curious and being more open minded, reliable and flexible.
check out career fitter dot com, SDS dot com ($10) and google 16 career clusters and explore careertech dot org (free) and read What Color Is Your Parachute (library) by Richard Bolles as starters. The best with this. Have FUN along the way!