If you don't already, subscribe to and start to follow cython-devel. First step is probably repeat the question there for more up-to-date info than what I can give (I don't even follow any longer).

Think about what you want to achieve/change in Cython. A new feature may be easier than a bugfix, though I'm not sure how many "low hanging features" are left at this point.... Anyway, make sure you understand what the change would involve in changes in the C code. Write a testcase that uses Cython the way you would want it to work (elicit the failure/bug/feature); look at the C code that Cython generates, and make sure you understand why that C code is the wrong code and that you know how you'd want it to look. (Understanding the generated C code at this level and read it almost fluently may take a little bit of getting used to but is an absolute requirement for working with Cython -- eventually you look past all the __pyx_ everywhere).

Then somehow try to beat the Cython codebase into submission in generating that C code... as you repeat this process you'll gradually learn the code.

It seems like Robert Bradshaw and Stefan Behnel are still around, they are very capable and friendly people and I learned a lot of what I know about programming from them, they were very welcoming to me as a new contributor.