To develop a Minecraft plugin you need some java skills, so here is a complete course in a single video.
And here I'll write a tutorial on how to set up IntelliJ:
Download IntelliJ Community Edition here.
Create a new project.
Now, you must go to the three lines at the top left, then to Project Structure, Modules, Dependencies, and click on the ''+'' symbol. Then, click on ''JARs or Directory'' and add your Spigot-API.
You can download the latest stable version of the Spigot-API here.