I am new on this forum. If I want to reply to someone's message, what is the proper thing to do so I can reply to a message posted on the forum? What icon or link do I click on to make a reply to someone's post?

Posts are grouped together into Articles/Threads. You just created a new thread, and I am filling out the form beneath the last post in the thread (in this case the thread only consists of your one post) replying to it.

Is that what you're asking ...?

I think at the time I was reading a bunch of threads and wanted to reply to one specific post. How do I do that if there is no reply screen showing?

You can only read a thread by actually clicking into that one thread and reading it, like here. The only reason there wouldn't be a reply form is if the thread has been locked due to abuse.

OK, I should be able to do that. Thanks for your patience.