Replying to a topic is just as easy as it is starting a topic. Once you have opened a forum and then opened a topic you will see what the topic is about (always the first post that the topic starter entered) and any replies that have been given. If you just want to reply to a topic you will see this at the bottom of your screen:
Clicking on the text 'reply to this topic' will open up the same editor that you used in creating your own topic:
Add your reply with the editor box and if you wish to be notified of any replies to this topic, click the button marked as such and when someone replies you will be notified; this is a great way of staying on point with topics you are interested in. Once you have added your reply just click the 'Submit Reply' button and your reply will appear within the topic discussion.
Any questions ask in the comments tab below.