Actually, that's covered very well in the FAQ (link above):
How do I find my way around?
When you click on a forum's name, you are taken to the list of threads it contains. A thread is a conversation between members or guests. Each thread starts out as a single post and grows as more individual posts are added by different users. Threads can be rated (?) to show how useful or popular they are and may contain polls (?).
To start a new thread simply click on the 'new thread' button (you may need the right permissions to do this).
However, thanks for posting since this question has been asked a few times before.