I teach environmental studies and environmental conservation from an ecological perspective. Very generally, a solid foundation in natural science, some quantitative and critical skills, a touch of computer programming experience, awareness of and basic literacy in economics, political science, sociology, and related fields, and sound English language skills (particularly in terms of technical writing) will all help in terms of preparation in college. You also want to develop applied and practical experience, and problem-solving ability that has some connection to your future career path so engaging in research, pursuing internship opportunities with connection to conservation, joining and leading relevant student organizations, and developing a sound understanding of your own motivation for that career path will all help too. Conservation and sustainability go well beyond my own area of focus, however, so you may want to explore the more humanities-focused side if that is a better fit for you. The good news is that whatever you choose to major in, your future career can focus on sustainability in some capacity; the challenge is that these inter- and multidisciplinary fields have many different options and choices for you as a student.
For most students, it takes some time to work out which path might suit them best, but as long as you choose experiences that will help you understand your own interests and passions. Keep asking yourself questions such as what are you doing, why are you doing it, what have you learned from that, and what can you do next to build on that experience? Continual self-assessment and reflection will help you as you explore further both in the classroom and outside of it.
Hope that's useful!