Source Tree Layout
An overview of the source tree may be found here.
Build System Basics
Detailed information about the build system may be found here; what follows is a quick overview, highlighting the areas where GHC's build system diverges substantially from the way make is used in most other projects.
Most projects keep the parts of their build machinery in files called Makefile found in many/most subdirectories of the source tree. GHC uses the filename ghc.mk instead; you'll find a file with this name in quite a number of subdirectories.
Other build system files are in rules/ and mk/.
The Coding style guidelines may be found on the wiki.