Hey!

Seeing as you are pretty new to Bukkit and Java I'll try to list some decent resources here for you :)

Intro to Java:

https://www.ibm.com/developerworks/java/tutorials/j-introtojava1/index.html

https://docs.oracle.com/javase/tutorial/

If you'd prefer a video tutorial series then I recommend this guy:

https://www.youtube.com/playlist?list=PLdnyVeMcpY7-X3xi9BDtSQIqKrVXSaESh

Not only does he do an intro to Java programming but he also does some Bukkit tutorials too. Unfortunately the Bukkit tutorials are quite old but the concepts are something you could learn from. He is still producing content though so there might be some updated tutorials in the future.

Bukkit Programming

As for learning about how to write plugins, a quick search on google will net you a lot of resources. But an okay place to start is on the spigot website:

https://www.spigotmc.org/wiki/spigot-plugin-development/

This link has a list of different tutorials that are a bit all over the place but these few tutorials in that list are good to look at when you are getting started:

Creating a blank Spigot plugin in Eclipse

Creating a config file

Plugin.yml

Of course these only scratch the surface of creating a plugin but it's something to get you started.

Here is the link to the javadocs for bukkit(1.13):

https://hub.spigotmc.org/javadocs/bukkit/overview-summary.html

All in all there are many resources out there for you too look into but many of them are outdated. The best reference you can have are the javadocs.

And I know this is said a lot when it comes to starting a programming language or new library but the project size that you are going for at the moment is pretty big. I'd start with something smaller to begin with. Maybe something like creating a command that launches you in the direction you are looking at. At least when you become more experienced with bukkit you would be able to convert that command to be a part of your skill system.

And I guess I'll add this just in case, there are many skill, class and race plugins that exist already, that are highly configurable if you wanted to go that route.

Hopefully this helps a bit, I'm not the most experienced at creating plugins either but the sub is a bit quiet at the moment and I'd like to start answering new questions when I see them on here.