Hello Jill. Thanks for writing! These are all excellent questions to think about.
Your interest in 'going green' is definitely something that relates to Environmental Engineering. But it also could relate to biology, architecture, politics, agriculture, or business! Did I confuse you yet? My point here is that environmental sustainability can be applied in so many ways. What's going to dictate HOW you put that passion to good use is what kind of person you are, and what you like to do. Are you a problem solver and into math and science? If so, engineering might just be the right thing. Are you more of a people person with good energy and wit? You might look into environmental public policy. Do you have an excellent memory and an analytic process driven mind? Perhaps its biology that you would like. Are you an artistic an observant person with an attraction to structures? You could be an architect specializing in Green design.
So, back to Engineering. If you decide engineering is for you, you will need to complete four years of college in an accredited college or university. Your first two years will probably consist of the basic engineering courses that all engineering students take including: three or four types of math, mechanics (static for structures and fluid for water/air), thermodynamics (relating to combustion), heat transfer (about energy transfer within materials). Environmental engineers will also need a good amount of basic Biology and Chemistry (some of this could be done in high school if you take AP classes). You will then get into the specific courses required for the major during your junior and senior years. There will be various offerings that you will be able to choose from, probably including courses on wastewater treatment, environmental remediation, ecological engineering, natural resource management, etc. I would also recommend three additional types of courses which may or may not be required - technical writing, geographic information systems (map making), and some type of computer programming.
Do not worry about gender distribution. It is not a factor that you should make any decisions based on. (If you absolutely want to know, environmental engineering classes will likely have 40 to 50 percent females, while all the other engineering core classes are likely to have more like 20 percent. Honestly, this does not matter an ounce).
Take Care and Good Luck!