Welcome Judy, from Ontario, Canada.
To reply to a message, you can either use the "quick reply" at the bottom of the screen, or if you want to use the smilies, just hit "reply" on one of the posts. If you want to quote a particular post in your reply, hit "quote reply" on the post you want to quote.
It's easy once you get the hang of it, and I'm sure you can get all the help you need here too, so don't be afraid to ask. :)