Well, it all depends on what type of learner you are. I learned by reading other people's codes, and changing bit by bit, until the whole code changed, from that I learned how to work on plugins. I would suggest learning the very basics, like If Statements, Variables, Functions and the rest will come as you code. I would also suggest searching Bukkit forums for coding help, there is almost 95% of the time a question asked, which you have too. Sometimes you could even see different style of coding, and learn new ways to expand your knowledge.
That 5% is mostly packets or something advanced.
Go on simple Bukkit plugin pages, and search for the Source, usually found on GitHub, and try to make a similar project to the one they made, and see how far you get, without using their code. Of course this is after you have learned the very basics.
This is the one of the easier methods to learn, other methods would be watching Youtube videos, or reading books about Java.
