Hi,
You is welcome to the game.
All the main starts to develop are typically available in web site :
The source code is located to the git KDE server. All developments must be done on master branch which will be the next 6.0.0 release.
Read well these files :
DEPENDENCIES
README.DEVEL
HACKING
With these documentation, you will be able to compile the code.
After, depending of which part you want to improve, we can guide you about the source code to patch.
digiKam is more than 1.2 M lines of code, but don't be afraid, all is well divided, sorted, managed. some documentation is also present in source code.
Best
Gilles Caulier