This guide will teach you how to get started developing civ server plugins. In order for you to follow this guide, you must already know how to code in Java. If you don't know Java, you should learn it before attempting to develop plugins in Java.
You will need Java, the Java build tool Maven, the version control tool Git, and a text editor you know how to use.
You should install these things using your package manager (sudo apt-get install
for Ubuntu) or download and install them in Windows.