A good place to start would be the Spigot wiki, which has a section with guides for developing plugins.
Spigot Wiki:
https://www.spigotmc.org/wiki/spigot/Plugin Development guides:
https://www.spigotmc.org/wiki/spigot-pl ... velopment/I personally use and recommend Netbeans for making plugins, there is a guide for that on the wiki, however a lot of the guides are built for either Eclipse or IntelliJ so you may wish to use either of those instead. I would also recommend Maven, which will download dependencies for your project for you (like the spigot API, for example). There is a guide for using Maven with IntelliJ on the plugin development wiki page above.
You will need to get the spigot jar, which for boring legal reasons cannot be distributed directly. You will need to build it using BuildTool. Read about that here:
https://www.spigotmc.org/wiki/buildtools/And if you're feeling up to it, Git is version control software which enables tracking and reverting history of code in a project as well as enabling various people to contribute code to a single project (even the same files, simultaneously with their changes merged). It can be a bit tricky to get to grips with, so don't worry if it seems a little daunting. However, I highly recommend it. Really helps to see what you've done in your last few code commits which broke everything in inexplicable ways
Git:
https://git-scm.com/You may wish to try some of the GUI clients listed from their download page. I'd recommend Sourcetree if you are running Mac or Windows.
You can host your git repositories for your project on sites like
https://github.com or
https://bitbucket.com for free, allowing others to see and contribute code.