Welcome to ATP, Roi !!
You don't need to "join" the forums, once you're a member of All Things Plants,, you belong (and are welcome) everywhere.
To reply, just scroll down the page to the "post a reply" box and type whatever you want to say. You can also click on "Quote" at the bottom of a message and it will take you to that "post a reply" box, along with entering the message from that person in a box within the message box.
Or, if you just want to follow a thread but don't have anything to say at the moment, go to the top of the thread, on the left side, and click on "watch."
Don't be afraid to ask for help, this is a very friendly bunch here... and there is also a "site talk" forum for questions about how to use the site in general.
