Since participating in forum discussions is the main reason to join a forum, replying to threads and posts is the primary communication activity for forum members. To create a reply in a topic, simply click the "Reply" button, which can be typically found in the top-right hand corner of the topic and you'll be brought to the posting page. For a breakdown of the features you can expect to find on the posting page, feel free to refer to the "Creating a New Thread" section of the help guide.
Another way to reply to a topic is by using the Quick Reply box. This box is typically found at the bottom of the thread, under the last reply on the page. You can simply type your message into the box provided here and then click the "Post Quick Reply" button found directly under it, and your reply will be posted to the topic.
Note that the Quick Reply box will only show if the forum staff have the Quick Reply feature enabled.