Study Physics, Chemistry and Biology, and Math in high school. Then at a University take more of the same and also study Geology and Oceanography. Marine Science is very broad. Are you interested in marine mammals, or microscopic marine life? Or chemistry of the oceans? Or their circulation? Or the rocks that form the ocean basins? Depending on the answer one would want to specialize in Geology, Chemistry, Biology or Meteorology.

As you may see Marine Science encapsulates many, many different areas of knowledge.

That depends on what kind of marine science you want to do. If you want to study marine biology, then you would go to college and major in ecology or marine biology (UCSB has both!). If you want to study physical oceanography, then you would study geology, oceanography, or atmospheric science.

After college you would probably go to graduate school, but without knowing more specifically what you want to do I can't advise you much further.

Having some idea this early is good! You'll want to take biology and chemistry classes in high school, and honors or AP versions when possible. Having math helps too. Then, apply to a college or community college with a biology program. They'll take it away from there :)