You must be registered and your account approved before you can post. You will receive an email when your account has been approved.
To post a new thread after your account has been approved make sure you are logged in. If you are logged in your user name will appear at the upper left of the screen, right below the blue menu bar.
Then navigate to the forum you want to post the thread in (Medicare General, Medicare Part A, Medicare Part B, etc). Then click the blue "Post New Thread" link which is prominently displayed at the top right of the forum. Follow the on screen instructions on that page and you have posted a thread!