It is usually wise to study the contents of a newsgroup before you begin. This will give you an idea of what is and is not acceptable on that group. Some groups have a moderator who keeps an overview of the group.
Read the newsgroup's FAQ
Resist overreacting to posts which contain strongly personal attacks, called a flame.
When you post your first question or response to another message try to keep your message concise and friendly.
You can respond to someone else's post by replying to the newsgroup or by sending them a private e-mail. Reply to the newsgroup only if what you have to say is meaningful to the newsgroup as a whole. In either case it is good practice to quote the relevant parts of the original message above your reply.
Some messages are sent to several newsgroups, called a cross-post. Cross-posts can be annoying as they often end up in groups which have nothing to do with the subject of the post. One variety of cross-posts called spams, can promote a variety of dubious and sometimes illegal schemes.
Be careful when replying to cross-posts. If you do not choose to send your reply by e-mail, or only to the relevant newsgroup, your own response could end up on dozens of newgsroups.