The great (and difficult) thing about majoring in Environmental Science is that it is SUCH a broad field. I had the same concern when I was studying ES in college but it turns out you can do next to anything with it, because you will have earned your B.S. and that says something in itself. You would be able to do anything from field work with animals across the globe, to environmental engineering/construction, to natural resource sustainability, to marketing and social media. The most important thing that you should start doing is make connections/ gain experience to pinpoint what you WANT to do, so that you have so many options.
Just start conversations with people and start making some connections, and definitely play around with the courses you choose to take in college, and you will be able to do pretty much whatever you want.