You can either use the Quick Reply box at the bottom of the thread, or click the word "Reply" located in the banner beneath the last post in the thread. That will open a box for you to type in. Go ahead and type your post. Then click either "Post Quick Reply" if you're in the "Quick Reply" box, or "Create Post" if you're in the regular Reply box.
In the regular Reply box, there are two tabs at the lower left. One that says "Visual" for what the post will actually look like when it's posted, and one that says "BBCode." I have to experiment some more with these to understand them better. For now, you're probably best off using the Visual tab when creating your message. I've noticed that sometimes you have to go back and forth between the BB code tab and the Visual tab to get a "preview."