This is a recurring question in the feedback mailbox, so I wanted to post some instructions regarding this...
Once you are logged in you will be taken to the home page. You will need to navigate to a forum, a list of which can be found beneath the "Featured Discussions" area. Once you click on a forum, such as "College Admissions", you will see a list of current threads in that forum. Displayed above these threads is a gray button that says, "New Thread". Click this button and follow the instructions. Or, if you prefer to join a current discussion, simply select one of the active threads to view all posts within that thread. Once inside the thread you will see a gray button that says, "Post Reply". Click this button and follow the instructions.
I hope this helps!