Success! I finally figured out how to reply to other people's posts on Twitter. (Click on the link in the upper left corner of the blog to access my Twitter page.) There's a little arrow icon, which I only just noticed, on the right-hand side of other users' posts. Click on it, and bingo! You're replying to that post.
Hey, I've only been using Twitter regularly for a few weeks. I'm still relatively new at this. ;o)