In some cases you have to be a member to post messages. This may involve payment, but it usually just means filling out a form and choosing a password and a user name. You might get sent some junk E mail in return, but that's what pays for all the free services on the web. There may be a second button which clears your information from the input boxes. You can often preview your message on the board, and change it, before finally posting it.
Some elaborate boards have several buttons for different purposes.
In the simplest forums your message appears on the same page as the input form, either above or below it, but in many of them the messages are posted to a separate viewing page. If you submit a message and it doesn't appear on the board, try clicking the refresh or reload button near the top of your screen. On the simpler boards, messages appear in the order they were posted. Often just a list of titles is shown, and you click on the ones you are interested in to see the full text. Sometimes you can search through past messages for a particular person or topic.