Please see this other post: Building a mental model of Blender code set
Also, your question is far too vague to answer. Do you want to help develop Blender? If so, which part/feature? What languages do you know? Do you want to learn to use Blender? Or did you want to develop Python addons for Blender?
Anyways, this is a good video to start with (It isn’t Blender): https://www.youtube.com/watch?v=ih20l3pJoeU