From your e-mail program, choose reply. Most programs will set up an addressed reply format for you, with the original text in the body of the reply. PLEASE, take the time to pull out the relevant subject from the original letter and delete the rest, then compose your reply. (Editing the original post saves bandwidth, and make it much, much easier for the rest of the list to follow the discussion.) After composing your reply, send it out and you've joined the discussion.