There are a few organizations out there dedicated to Green Building. I'm just listing a few here which I found most useful:
The US Environmental Protection Agency has a website with good information about Environmental issues, toxic substances, environmental regulations and funding for environmental initiatives. It is a great place to start researching the basics of Green Building.
The US Green Building Council has a site which promotes education and the LEED building rating system, a benchmark for green buildings
The World Green Building Council is a union of a number of national green building councils andorganizes international congresses and the like.