The forums are designed to be as simple to use as possible.
Imagine being in a hotel... there are lots of rooms and different things happening in each.
Each "Room" is a category within the forum.
Within each room (category), there are groups of people, having various conversations (threads) with people making comments (replies) as they see fit.
In order to join in a conversation you simply click on it's name, to see what people have said in it, and then create your own comment and add it to the conversation.
It may be that you want to start a conversation of your own.
Simply decide on which room is most suitable for the conversation, go to it, and press the "Create Thread" button at the top right of that area.
Give it a subject, put your comments in the main area and then press the "Create Thread" button a the bottom right of the screen.
This will make your conversation live, other people will see it and, hopefully, start commenting on it.
Likewise, you can view other conversations and post Replies at the bottom of the thread. You can also add smilies (like this one by clicking on the icon at the top of the editor window and picking which emoticon matches your comment or emotion. You can also add attachemnts, poll surveys and form your comment/thread as much as you like using the toolbar at the top of the comment window.
Have fun, and shout if you need help, either on here, or by tweeting me.