You compose and reply to messages in a newsreader exactly as you do in an email program. The only differences are in the message header, because instead of addressing a message to a person, you're addressing it to a newsgroup.
The easiest way to deal with that difference is to start in the right place. For example, when you want to compose a new message (not a reply) and post it to a newsgroup, begin by opening that newsgroup, and then clicking your newsreader's button for composing a new message. (It's New Msg in Netscape Messenger, New Post in Outlook Express.) When the message window opens, you'll see that it's preaddressed to the currently open newsgroup.
When replying, open the message to which you want to reply, and then click the Reply (or Respond) button on the message window in which that message appears. In the message window that opens, the message is preaddressed to the appropriate newsgroup, the subject line is correctly phrased to add the reply to the same thread as the original message, and the original message is quoted in the message area (see Figure 7.8). Just add your comments, and edit the quote as necessary.
Figure 7.8. Start a new message or reply while viewing the message list of a newsgroup, and that message is preaddressed to the open newsgroup.
After completing a new message or reply, send the message by clicking the button or menu item labeled Send or Post.