These tend to operate on an American concept of profanity, so if you want to talk about your four legged friend with long ears, you'd better call her a donkey instead of an ass. You needn't worry too much about spelling, grammar or punctuation, nobody else does. In fact English seems to be a foreign language to most people who write stuff on the web. It's easy to be misunderstood when the expression on your face can't be seen, and people seem to get offended more easily in cyberspace than in real life. Each forum is usually dedicated to a particular subject, which may be quite broad or very narrow.
Some sites have several boards for different specialised topics.
You won't be popular if your contributions are irrelevant or posted on the wrong board. The biggest sin of all is to send the same message to a lot of different boards, which is usually done by people trying to sell something. This is called spamming, after a Monty Python comedy sketch involving a cafe in which every item on the menu was served with spam, spam,spam and spam. So now you're ready to join in or start discussions. If you want a topic to discuss, read the online articles I've listed on the links page.