Your first “real” posting can be a follow-up to a previous one, or a brand new query or comment. If your posting concerns a message that has already appeared, it is preferable to make sure it's included as part of the relevant thread.
Your message, also called an article, contains headers followed by the text of the article. Each article is directed to one or more newsgroups by a Newsgroups line in the header. You can send your message to more than one newsgroup at once, but this is sometimes considered a breach of netiquette.
A follow-up article has the same Subject line as the article which inspired it. (The notation “Re:” is added.) This makes it easier for other users browsing the newsgroup to follow the discussion. Typically, a follow-up article selectively quotes the earlier article–most newsreaders do this for you automatically–to make the discussion easier to follow.
If you ever send a posting that you regret later, you may be able to cancel it. Some newsreader software lets you attempt to cancel a message, but news servers are not obliged to act on this request. On many servers, postings routinely disappear after about a week. However, postings are not removed from the Usenet archives, and may well linger in other archives too.
If you don't want your post to be saved for posterity, use the x-no-archive command. Simply put the following header in any post: x-no-archive: yes
To do this, you can use a command built into some newsreader programs, designed to let you enter x-headers directly. If your newsreader doesn't allow this, just type the command above as the first one in the body of your newsgroup posting.