If the person asks you a question about something you posted on the general forum, you just come down to the box that says "Post Comment," reply and then click on "Post Comment." It will show up on the forum in the thread it was posted under. If they have sent you a PM (Private Message) which you would be notified about in your home email, when you click on the link, you will see their message and you reply to that the same way you do here, with the exception that it will be a private message between you two only.
Did that answer your question? If not, let me know and I'll try again! I'll send you a PM and you can practice on me, OK?
Peace
Greenlydia