If you attend a high school that has a journalism program with courses tied to the area you'd like to pursue (TV, Radio, writing/digital media), that would be the most direct path. If this isn't an option, a couple of alternatives would be taking English and/or literature classes if your focus is on writing.
You should also look into programs away from school. There are different organizations -- perhaps a local newspaper/TV station/radio station or professional journalism groups that -- oversee student projects.
Whatever your area focus, you should constantly work on your craft. If you want to write, practice writing stories and columns relating to current events. If you're looking to work in front of the camera or on radio, do practice recordings and go back to watch and/or listen to yourself. Have others critique your work.
One thing to remember: There are so many job roles and opportunities in this business (just like any field). My answer centered around talent positions (writers/reporters, TV anchors, radio hosts) but there are a ton of behind-the-scene roles (editors and producers -- the people who help shape the stories and coverage you see on TV and on websites). Scrutinize your desires and talents -- is your focus on reporting/delivering the news or helping shape it from a planning perspective? Either way, it would help to find mentors in the industry who can help guide your path.
A journalist must be able to think critically and communicate effectively verbally and in writing.
Any courses that help you develop those skills will be valuable to you as a journalist. For example, courses in the social sciences tend to require reading a variety of materials, analyzing them and presenting your analysis in an essay; same with courses focused on the humanities. Sciences tend to reinforce critical thinking skills and are also valuable.
Additionally, it's is useful for a journalist to have a broad an education as possible... you want to be exposed to many different subjects to help develop critical thinking stills along a variety of areas—everything, say, from biology to literary analysis. The more you understand and the broader your education, the more effective you are likely to be as a journalist.
For example, I worked in journalism for many years but I've never taken a single journalism course in my life. I pursued a liberal arts education, honing my ability to think critically and communicate effectively, and learned "the ropes" of day-to-day journalism working in newsrooms.
Any good editor can teach you the nuts-and-bolts of journalism. What she/he cannot teach you is to think, ask questions, analyze data and present it coherently orally in written form. Learn those skills in school and you'll be in strong shape for a career in journalism.