I want to contribute to CodeIgniter!

[eluser]solid9[/eluser]
Hi guys.

As a CodeIgniter user.
I wanted to help improve this great framework.
But I don't know how the inner of CI works.
Any tutorial?, or Documents?,
or anyone would like to explain to me,
how the inner CI engine works?
I know how to fork it from github and push it back.

Thanks again in advanced.