I’m going to recommend you take a course, read a book, or at least go through a good tutorial. (FYI I have some online courses available). I worry that by learning from this old code base you are likely to pick up misunderstandings and unnecessary habits (i.e. Cargo cult programming - Wikipedia).
You should make sure you have a firm understanding of things like
- how source code is organized
- what the flow of execution is like
- variable scope
Imagine trying to learn a spoken language by reading a story you’ve never heard before and looking up the translations of individual words as you encounter them. There’s a lot of possibility for misinterpretation. Same thing with learning a programming language by reading a program you’re unfamiliar with and then only asking about specific aspects.
Of course do come back here and ask questions if the material you choose to learn from leaves you confused or doesn’t explain something in enough detail for you.