What do I need on my XP in order to develop and build a plugin?
Openfire Source, Java JDK, Ant Buildtool
What kind of JAVA do I need on my computer? JRE?
No! You need the JDK (Java Development Kit)
Do I need eclipse?
No you don’t. Myself is developing just with a good texteditor and some console windows, but using linux, not windows. But I think most people are not familiar with the workflow and speed advantage using only a console, so you maybe want Eclipse…
It says in the guide to use Ant. Is it part of eclipse? How do I do it?
As far as I know, no. How to configure it is described in the guides.
Could you please instruction for the most simple plugin?
Take a look an sourcecode of some of the official plugins. The presence and the search plugin are a good choice.