New To Java - Want to start a big project, how to start?

Hello! I for a little knowledge about myself i have been tweaking with java, Unity, and swift coding over the last 2-3 years but none of the knowledge retained. However i am very commonly used to plugin configs as i am a admin on a server and i run a server of my own.

I want to make a plugin or two and start devloping plugins that are usefull toward both the community and myself on my own server that would add a class base and race base system with a custom skill GUI thing.
I like going head first into big projects, its just how i have been able to learn a lot easy for me to learn.

I was looking into other plugins that do similar things however they are outdated or the devs are very very very slow in response in which i think this is something mine craft owners need or some anyway.

Any ideas on how to start and any ways to make a gui thing possible. Or videos, tutorials or blogs?

(1.14.4 version) Thanks to anyone that responds! I want to learn and learn anything i can!

 

@stormy_bluey Here is a few links that might help with the GUI thing you wanted.
https://www.spigotmc.org/threads/tutorial-how-to-create-a-gui.280215/
https://www.spigotmc.org/wiki/creating-a-gui-inventory/