Don't worry, this was my first forum and I had no idea what I was doing when I first got here, either.
It's pretty straight forward. There are always a lot of discussions going on, and feel free to jump in for any of them. If you want to start a new topic on a subject, it might be better to use the search bar (top right) to make sure that that topic doesn't already exist before you do. Sometimes multiple ones will have already been made on the subject!
Otherwise, just join in. Everyone here is pretty nice.