So the classes you take in high school don’t really decide your major in college. For something like Oceanography, you would certainly want to take science classes or any class directly related to the ocean. Though these will just give you a better background and won’t be the deciding factor on what you end up majoring in. In fact, many people would say what you major in doesn’t even have a large impact on what your eventual career is. Generally speaking, in high school you should take the classes that interest you the most.

In high school:
Math (up to calculus if possible, trigonometry if not)
Geology (if your school has it)
Physics (preferably advanced placement)
Biology
Chemistry

In college:
Oceanography (likely its own subject)
Atmospheric science/meteorology
Geology
Marine biology
Physics

Oceanography is a multidisciplinary science. This means that to be a smart oceanographer one needs to have a broad background in science and math.

So, one should study two years of Chemistry, two years of Physics, two years of Biology and as much Math as you can tolerate. This material becomes the background.

Then, depending on what ASPECT of Oceanography you do research in, one needs to acquire additional knowledge in Geology, fluid Mechanics, Meteorology and data management.

But while you study all this stuff, you should simultaneously be reading general Oceanography textbooks. From an introductory textbook you can come to learn of all the sub disciplines within the GIANT field of Oceanography.

Beyond the basics that all high schools require, take a lot of additional science and math courses. There are a lot of different focuses in Oceanography. Some people study the animals, others the currents, others the effects of temperature changes, etc. But regardless of what your focus will be, you will likely need a good background in Math (algebra, calculus, etc.), Chemistry (the ocean is all about things mixing, pH, etc.), Physics (the ocean is constantly moving, along with the things that live in the ocean), and Biology. With a good understanding of these topics from high school, the college introductory courses will be easier and you can more quickly get into more advanced topics that align more with your specific interests related to Oceanography.

It's great to hear that you're interested in oceanography! In general, if you plan to become a scientist, I would suggest becoming well-versed in statistics, learning how to communicate your ideas well, and learning how to learn (figure out how you learn best!)

A more specific answer to your question would probably depend on what kind of oceanography you would like to do. For instance, typical subjects of interest for biological oceanographers (aka marine biologists) are the ecosystems, behaviors, and effects of pollution on animals and plants in the ocean. Some relevant classes may be various biology and chemistry classes. On the other hand, a chemical oceanographer is interested in the chemistry of the ocean and how pollutants might affect the various ecosystems of the ocean. Often, chemical oceanographers are also interested in the effects of atmospheric and terrestrial chemistry on ocean chemistry. Some relevant classes might include organic, analytic, atmospheric, and oceanographic chemistry, as well as other basic earth science courses. Physical oceanographers are interested in the physical aspects of the ocean, such as the relationship between the ocean's currents and weather patterns. Physics courses such as fluid mechanics/dynamics, thermodynamics, and a good amount of math (calculus, differential equations, etc.) would probably be good to take if you're interested in this field of study.

Ultimately, what I would suggest to you is that you develop as many transferable skills as you can. For example, most scientists need to be able to work with large amounts of data, so getting comfortable with data analysis is a good skill to have in your toolbox (this includes being comfortable with the software and perhaps even small amount of programming you might have to do to carry out data analysis). I hope this helps!