I'm late to the party but anyway.
First ... did I say Welcome to the forum yet @GrahamB? If not ... Welcome to the forum first.
Then, just to mention, that a lot of useful "how to"s you are ablle to find here. It is at the Welsh side of the things but can be generally used for the whole forum. The catch is just to be at the right place, what means if you want to post in "other languages" thread you have to "be" there. If you want to reply to one topic, you have to be in that topic and use reply button (as @hectorgrey says) at the end of the thread. If you need ot reply to particular poster, you hit the Reply button under that particular post.
I hope this and all hints in the suggested topic can help you navigating through the whole forum and do the stuff you want.