If you work at Google, you probably want to read the Google-specific instructions instead, which are basically the same except for some details relating to the Google corporate computer images.
Chromium supports building on Windows, Mac and Linux host systems. Linux is required for building Android, and a Mac is required for building iOS.
Click on one of the following links depending on what you want to build.
The Chromium documentation is gradually moving into the source repository. The links above will take you there.
If you need help, try the chromium-dev Google Group, or the #chromium IRC channel on irc.freenode.net (see the IRC page for more on how Chromium uses IRC). These are not support channels for Chrome itself but forums for developers.
You can also, for a limited time, read the old instructions for getting the code.