What should I do if I don't know what I want to study?

A person's interests and passions can change over time, so first of all, don't stress about this. The exploring will be fun! In addition to what Phillip mentions above, I would make a list of the activities and subjects you are even slightly interested in and try to understand why you like them. For example, if you like your Literature classes, is it because reading is enjoyable to you or because you like writing the assignments or you're good at presenting in front of your class? Or maybe you enjoyed reading a book about Science a lot more than a novel. What extra curricular activities do you enjoy? If you come with some of these in mind or some initial thoughts on them, the conversation with the counselor will be much more fruitful.

Good luck!

Great ideas from both Phillip and Ana above. i would also suggest getting a copy of the book "Strengths Finder 2.0" from Amazon. It is fairly cheap and will assist you with identifying what it is you are good at. You can then use those items to assist in your discussions with the councilor to help nail down some ideas. It is true what Ana says about passions, likes and dislikes changing and evolving over time. Don't get hung up in the idea that they are carved in stone. If you find something you like, do it for a while and then find something you like more, be willing to change and explore the new area.
You've heard the term "follow your dreams". That is interpreted by many to mean a single dream and that leads to the idea that you have to identify it up front and then chase it for the rest of your life. What it is really saying is to be flexible and follow what you discover you like and are passionate about. Don't tie yourself down to one thought or idea. Doing so just robs you of countless other opportunities.
Best of luck to you!

You should go see your guidance counselor to see if they can get you an aptitude test to determine what field of study would best fit you.

Hi Mark!

Here are some fun ways to help determine your passion.
https://www.themuse.com/advice/the-most-useful-personality-quiz-youll-take-this-week
https://www.themuse.com/advice/14-free-personality-tests-thatll-help-you-figure-yourself-out