Start coding for Blender
From Wiki.OSArch
Blender core is coded in C. Some part like Cycles are coded in C++. Python is widely used for extending Blender functionalities with scripts and addons.
(For BlenderBIM Addon specific code see BlenderBIM Add-on code examples)
C/C++
Blender foundation made a video to present code structure :
python
Introduction
Good introduction to python use in Blender from Curtis Holt :
To learn more checkout the Blender Python API documentation especially Quickstart part.
Tips and Tricks
Blender Python API documentation has a special Tips and Tricks chapter including external editor usage, inserting a python interpreter etc…