I've recently been asked for advice on writing a FileNet application using the Content Java API. To be honest, I'd be surprised if there aren't already a plethora of tutorials on this subject, and it's not exactly rocket science.
I must confess that for some years I used the "classic" Java Development environment - Windows Notepad, DOS batch files, and the command console! No excuse really. I have a background in Windows development, using VB6 and Visual Studio.net, so I didn't need anyone to persuade me as to the productivity boosts in using a professional IDE.
However a few years ago I finally bit the bullet, and decided to use Eclipse. I wrote a few notes on this, and so have uploaded these - take a look here: Using Eclipse in FileNet P8 Development.
The official online help also gives enough information to get started: FileNet developer help.