The last time I tried to reply to someone's thread, I had several comments in the box before I put my reply. How to I reply?
When you're in an active thread, you'll see a button under the lower left corner of each post and a button under the lower right corner. If you just want to add a comment, use the button, if you want to comment specifically to something someone else has posted, then you use the button. Starting a new thread is rather interesting and something you'll have to figure out on your own because it's rather lengthy to try to explain.
Last thing is that you generally don't have to be logged in to make a post, however you don't have the same editing and functionality when posting as when you are logged in.
Hope that helps. :)