To post in a thread, simply hit the "add reply" button on the bottom right of the post you are reading. Type your post in the box and hit the "add reply" button on the bottom.

To start a new thread, click the "new topic" button, and when the box comes up, make sure to enter the "topic title," (topic description not needed), and post what you want in the box and click the "post new topic" button.

Simple!