Go to the board where you wish to post and click on the New Topic button (positioned by default at both the top and the bottom of the board), which will take you to the Start New Topic screen. While this presents a number of options, the two most important are the Subject field and main text area for the message itself. Enter your subject and start typing (or paste) your message in the main text area. Once you are happy with your message, you can post it by clicking the Post button and/or preview it first by using the Preview button. Some forums may be configured to offer an additional Spell Check button alongside the Post and Preview options.