Reply to a Thread
First, go to the thread to which you want to reply.

Then, when you are viewing the posts in that thread, you have multiple ways to reply.
You can click the New Reply button at the top right of the thread, just above the first post, or at the bottom right of the thread, just below the last post on the page. Type your content, using the menu options as needed, then click Post Reply.
At the bottom of the page is a Quick Reply window. It offers fewer options, but you can type your reply in that window and click Post Reply to post it.
Under any post in the thread, find the Reply button. Click it, and a Post a New Reply window will open with the text of the post included as a quote.