Posting to a Topic
To post to a topic in a particular forum, scroll to the bottom of the page, where you will find the form labeled Add a Reply. Type out your message, mark it up with BBCode or HTML as you prefer — only basic HTML markup, though, such as STRONG, EM, A, BLOCKQUOTE and so forth is allowed — then hit Reply. It’s just that easy.
If you want to reply to a post you see in the activity stream, click the View link in the activity entry to visit the topic page, then scroll down to the Add a Reply form, just like above.
Following Forum Topics
Following a topic means you can be informed of when someone adds a reply to a forum thread without having to manually visit the page in question to check. There are two main ways of doing so:
Every forum topic has a button at the top and bottom of each page, labeled “Follow this topic.” Click that button, which then changes to “Mute,” and you will receive notification via email whenever a reply is added to the thread.
You can subscribe to any topic thread or forum by clicking on the adjacent orange RSS icon, or pasting it in your reader of choice.