You gotta start with the Pope if you are going to talk about the Church as a whole, and if you are gonna start there, you should probably start with JPII because I doubt the environment was very much on the radar (or certainly not the kind of environmentalism we think about today) before his time.
This link:
http://conservation.catholic.org/pope_john_paul_ii.htm looks like it has some very good starting points. You'll want to pick one angle from the many presented there and run with it.
I'd shy away from the simple notion of "God gave man dominion over the earth", it's easy and not very enlightening. Build a line of thought about how we discover God in the various forms of creation, and thus discover ourselves, or examine the tension between environmentalism and the pursuit of greater human dignity and social welfare (Redemptor Hominis). Heck, you could even examine the role of environmental stewardship in liberation theology in developing nations.