Start by learning about the Python C API. It is a large and rich API, and the Python source naturally uses it all over the place. You won't get very far into the Python source code before you have to understand what is meant by Py_INCREF and so on.
I gave a presentation at Pycon explaining the API: A Whirlwind Excursion through Python C Extensions that you might find helpful. C extensions use the same API as the Python code itself.
