You need to first establish exactly what you want to do, and then consider how you want to do it. Creating an IDE is a complicated proccess, so you may want to think through thoroughly what functionality you want to include etc....
I assume by java IDE, you mean an IDE that works for java, rather than an IDE that is necessarily created in Javanese, so you may consider prgramming it in a different language.
As with any project (and certainly any of this magnitude), you might want to follow some form of SDLC, to plan your design beforeyou go along and try to implement.....
If you have questions about a specific aspect of the IDE, by all means post again, but because of the scale of such a project, it is difficult to point you in the right direction as it is not clear what you want to do.....