This can be anything, but obviously it should be your real name. Some sort of IP-address/name-binding may be possible in future, to simplify filling this information in, or recent names might be logged and used in a pop-up menu, but for now, this is where it goes.
The topic text is usually used as the title and header for the new discussion thread's page. It should be meaningful, while short enough to avoid line-wrap if possible.
The large window of text will accept typed in text, and should "autowrap" when you get to the right margin of the window. This information is passed back to the browser to be displayed as either HTML or plain text, depending on the setting of the (optional) radio button (HTML is the default).
If you want to enter a couple of paragraphs, you might want to put paragraph tags (<p>) at the end of each paragraph.