Major: Any major will do! --> Pick one you like and do well in.
Add Supporting Courses: Such as these listed HERE. They will complement your major and add credibility to your interest in an environment-oriented career. In addition, each time you have the opportunity to choose a paper topic, choose one that is environmentally- or sustainability-connected. Here, you are "self-teaching" -- which is what most of us do all the time in "the Real World" :+)
EXPERIENCE: Participate in activities like EcoHouse, internships, clubs & activities, part-time jobs,
and Study Abroad. Worried about not knowing anything? Try volunteering! In the end, employers and graduate schools don't care whether you were paid or earned credit; they want to know: Can you do the job? Most students underestimated the value of experience; don't be one of them!