This article explains how to create a simple Eclipse plugin. To understand plugins, the easiest thing to do is to use the built-in wizard in Eclipse. You need the JDT to have the wizard I believe (you can also download a RCP distribution of Eclipse). Here are the steps:
I am happy to announce that I will serve as a mentor for two Google Summer of Code 2015 Eclipse projects, namely, Convert to a lambda expression Java 8 refactoring/quick fixes by Md Arefin (poster), and JDT UI: Adding New Refactoring Features and Improving Code Completion by Gábor Kövesdán. Google Summer of Code is a global program that offers students stipends to write code for open source projects. Congrats to Md and Gábor!