When someone posts something to a newsgroup, that's the Internet equivalent of writing something on a piece of paper and tacking it to a large corkboard posted in a public place. Other people will then comment on what you have written, in the same sense as if they tacked up their response to the corkboard, right next to your original article. There are many different kinds of newsgroups on Usenet, and the first section of the newsgroup name should provide a clue as to what kind of newsgroup you're dealing with. On a newsgroup, you write an article and people write followups. Newsgroup threads are very complex things for a number of reasons.
Posts can be articles, followups to articles, or followups to a followup.
But sometimes they won't, and several unrelated conversations will occur beneath the same topic header. It's important, however, to have some idea of what you're doing. Now, on to the basics. And they'll know if you do something stupid. And they'll tell you.