Where do I find information about setting up a build environment, getting the right tools, and actually building the source?
Start by reading the release notes for the build, these can be found in the source download and on the Downloads page. The release notes tell you everything you need to know about this specific release of the OpenSolaris source code, including software requirements for building the source. The Developers Reference document should answer most of your questions around building the source including details about the tools you need. This document will also explain how to update your system with the latest OpenSolaris binaries, and just about everything else you need to know as a developer.