Click the Forum link on the left hand side.

You will see a list of Forums and Topics within each Forum. For example, above there are two forums and one topic within the last forum. You can only post messages within topics.

Click the Topic that you wish to post in (e.g. Introduce Yourself). You will see a screen similar to that shown below.

Type your message to post.

Finally, if you should see your posted message (that others can see also.