Once you log in you will see a list of all of the available forums. You will also notice that the welcome message welcomes you by name. Click on the discussion forum you want to enter. This will bring you into the forum you selected. Some forums are organized with sub-folders, as seen in the example below.
Folders just make it easy to organize materials within a forum.
You will read the topics and replies, and reply to it. When you click on an entry, you will see the actual message which has been posted. This shows the topic view, including the text of an entry. This will bring up a form, where you can enter a title and the text for your reply. Otherwise, just leave the formatting set on Automatic.