Hi Dave. If you click on the FAQ link (Frequently Asked Questions) at the top of the forum, it will answer a lot of your questions. Many people aren't even aware of the fact that this is even there.
But to give you quick answers to the question you posed, you are already registered and logged in. So that is a beginning. Anyone can read from the forum, but only those who are registered and logged in can post. This was done to prevent anonymous postings which happened at the beginning.
When you click on a post and want to respond to it, look at the links that are at the very bottom right of every post. There are two in particular that will be of interest to you. There is one that is titled "reply" and one that is titled "reply with quote".
When you click on either, a message box will come up and all you need to do is place your message in the box and then click "Post Message" at the bottom beneath that message box.
"reply with quote" will quote the previous poster's message above yours. Simply clicking on "reply" will allow you to post a message beneath the other's without his quote appearing there.
You can also edit the quote if you only want to respond to a part of what was previously posted, and you can click on one of the little "smilies" that are grouped to the left of the message box before clicking on the "Post Message" if you want to add one. ( Like this

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If you want to see if everything is well before posting it to the forum, click on "Preview" first. Otherwise just click on "Post Message". You have a 50 hour window in which to edit the message if, after you post it, you see a typo or wish you had said something slightly different.
Try it. Play around with it. And have fun.
Oh, and welcome aboard. We were all new once, and we still don't know it all. That is what this forum is all about.