Are you talking about the built-in editor that comes with WordPress? Or the Write Post Panel where you write your posts?
Okay, here is information on both.
How to edit files in the two build-in editors that come with WordPress (one is the File Editor and the other is the Theme editor but they both do the same basic thing. http://codex.wordpress.org/Editing_Files
To understand how to write a post and more about the Write Post panel:
If this isn't what you need, let us know.