You post a message to a newsgroup when you reply to an existing message, which adds your reply to that thread of discussion. You can try composing some test messages and replying to them in the test groups.
You can reply to a message at the top level of a thread, starting another second-level branch, or to one of the subordinate messages in an existing thread. Many people reply to the last message in the first branch, creating one straight branch in a long line. Other messages enjoy healthy growth in several branches, such as in newsgroups covering controversial topics like the “talk.*” hierarchy.
You can reply to a message by selecting the “reply” menu item, which opens a new message in reply similar to replying to an email. Your software will usually “quote” the original message by including it in your posting. If the message is already part of a thread, there will be several levels of indentation of previous messages in the thread. It is considered good form to delete all of the previous messages except the parts that you want to reply to, often replacing the deleted portions with the word “<snip>”.
You can put your reply “inline” to the previous messages, by adding your replies to the individual points of the previous messages by simply adding new lines at appropriate points underneath the text that you reply to, making inline threads of commentary like the following:
> About this tomato plant, how tall does it grow,
About 3 to 4 feet
> and can it take a lot of sun?
It loves sun, but doesn’t mind a little shade.