Just went to a marvellous talk by Jane Goodall here on the USC campus, in Bovard auditorium. She’s signing her new book as we speak! Among the many things she said, she emphasized one of my favourite themes with regards the environment (and so many other things, like community, education, etc): Act locally.
Have a look at her roots and shoots organization for example. It is very youth driven too, so might be a really interesting activity for someone you know looking for a new organization to be part of that can make a huge difference.
There might be a branch (sorry) near you.