Posting a message on the forum can be achieved by creating a new topic to share and discuss something of interest, or a forum post can be a reply to an existing thread created by someone else.
They are both the same mechanisms so using one means you can use the other, but they have button links specifc to each case.
1. Creating a New Topic
The first thing is to decide where your new topic will go in the forum. The forum has various categories to keep like topics grouped together. Some parts of the forum might be visible but private, depending on your access privileges such as rank.
For example doing an introduction post as a new member if firstly about finding the Introduction part of the forum, its called "Introduce Yourself", and then clicking a button called "New Topic";
This takes you to the standardised forum post page where you can put in the topic's title of your choosing, like a subject heading, and the body of your post itself.
2. Joining in on an Existing Topic
When finding yourself reading a discussion where you want to add some of your own thoughts, you have two options, you can either 'Reply Topic' or 'Quote'.
The most usual thing to do for generally joining in on a dicussion is to use the reply button, and then fill out the fields. You'll see there are some options such as adding a picture. For more advanced or curious users there are the BB Code menu's above the reply content field to add formatting and links.
Once you've had some practise at these, try to reply using the 'Quote' button instead. This will allow you to reply to specific points or to clearly indicate who you might be addressing with your reply.
This is just a brief overview, but it should help you get started on joining in at the forum... happy posting!!