Just like the environment, the world of environmental studies is very interrelated and gives you a whole lot of options with where to take it. Pamela already gave great advice--it really is about what attracts you, but I know it's hard to narrow it down if you're passionate and would be happy in more than one path.
My experience was that I knew I was worried about coastal wetland loss, I loved animals--especially marine animals, I was worried about climate change, and I believed in the power of people to solve these difficult challenges. I studied Environmental Science (and English) in undergrad to get a broad sense of the subject, then I worked in environmental non-profits. For me, the work was important but I felt like I wasn't making the impact I wanted, so I pursued a professional master's of science in environmental management and sustainability. That program helped me see different paths of using my passion and knowledge of environmental science in the professional world, my peers from the program are doing a number of things: one opened her own business providing micro-loans to under-served communities, two work in energy audits & efficiency, one works in environmental policy in Washington DC, another works in sustainability consulting for an architecture firm, one manages sustainability projects for one of the US's biggest airports, one works for disaster relief on the East Coast, one's a geologist looking at environmental health. I work in corporate sustainability in the IT sector, helping set effective sustainability development targets for our company like improving the energy efficiency of our products, switching to 100% renewable energy, etc.
Learn as much as you can about environmental science and its different applications and find what speaks to you and what path might make the best use or your interests, strengths and perspective. Don't be shy to reach out to people who might work in the area you're interested and learn a little more about their day-to-day.
Best of luck!