After signing in, you'll see you're large "M" avatar in the Upper right of the page. To the left of that is a "new" icon. Clicking on the "new" will bring you to the latest/newest discussions.
When you have something to add to a discussion, go to the blank area under the post, type, then either click on "preview" or "post reply" to add your content to the discussion.
You can continue to do that, or go to the top of the Forum's page, and click on "Home> Forums>" to pull up a more detailed list of categories, hopefully sorted by content and title, that hold previous discussions.
Play around and get comfortable. You can't mess up anything.
Just be mindful, there is a mixed and diverse group of folks on the Forum. Keeping things civil would be helpful and appreciated.