Stanford Earth, the School of Earth, Energy & Environmental Sciences, offers four undergraduate majors and a myriad of introductory courses. Here are some suggestions for getting started in Earth, Energy & Environmental Sciences that address both classes and summer opportunities. Depending on your particular interests, you may want to take chemistry and calculus as a first-year, as those may be prerequisites for other courses that interest you.
Stanford Earth has a wonderful field program with opportunities open to majors and non-majors alike. Students have gone on field trips to Death Valley over Thanksgiving; Joshua Tree over spring break; or Owens Valley over Memorial Day. Keep an eye out to see specific times and places announced each quarter.
Stanford Earth also has a strong program supporting undergraduate research, and many frosh find opportunities there for their first summer. The school also runs a quarter in Hawaii program every other year.
If your interest in the environment leans toward the oceans, be sure to consider visiting the Hopkins Marine Station.
If you're passionate about sustainable agriculture, take a class at the O'Donohue Family Stanford Educational Farm