The Development Documents are a good place to start. These will help make sense of the code you are looking at to begin with. When you are familiar with coding standards, I think it might be of benefit if you were to download an older, somewhat neglected plugin, and bring that up to current standards. Won't matter what it is, just something that was used but now has no-one to maintain it. Nothing too old, and as it is mostly Open Source, you don't have to tell anyone you have done it, don't have to share it with anyone, but it may provide experience for you.
Practice with that, get it working, then you will have a better idea of how to get your own project going. Don't spend too much time on it, that is not your primary target, just enough to give you some experience.
Specific coding questions are likely to garnish better responses in the General Developer's forum. Good luck and I do hope it works for you.