How does one getting started developing for spigot?

Do you know Java?

 

Before you even touch the Spigot API I'd learn Java first. If you don't know Java you should take a look at thenewboston's tutorials they're really helpful and how I got started out. You can also pick up a book or two, I don't have any recommendations but you can find some on Amazon for around $20.
Once you know Java check out the Spigot Docs and this really helpful tutorial. That's all you really need, if you have any trouble you can always post on here and people would be happy to help

 

Basically you have to Learn Java first so you understand some really important things, and then its basically just an API that you use to create some fancy features
But note: Learning Java can't be done within some days, this could take years when you don't keep like every day learning.
Learning Java is basically the same to learn like another language than your native language ^^

 

Once I learned java I looked at and edited projects by other people and learned about the Bukkit API based on what they did.

 

https://www.spigotmc.org/wiki/spigot-plugin-development/
I didn't even know this page existed until I used this marvelous thing called "Google"

 

if(!Java.hasUserKnowledge) {
user.startLearning();
} else {
user.listSpigotJavadocs();
}

That´s everything what I can say

 

Error: Field 'user' is undefined.
Warning: Missing indentation.
Warning: Field 'Java.hasUserKnowledge' is not user-specific, use encapsulation instead (User#hasJavaKnowledge).

 

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I suggest you to start firstly by learning Java. Do some practice by developing some simple programs such as a calculator. When you learned enough start to follow some Spigot tutorials.