I notice from the list of your posts, Wishing, that you've successfully started new threads as well as replied to existing ones so you're apparently able to do both. I'm guessing, then, that either the button that you were looking for wasn't where you were looking for it, or it was labeled differently from what you expected, or it did something you hadn't expected.

I found out early on that for almost everything I might want to do at this site, there are two or three ways (at least) of getting to it. If I click on the "wrong" button and get a text box with a quote from someone else's post, I can delete all or part of the quote before typing my reply. If I was expecting a quote but I just get a blank text box, I can paste the quote in myself, etc. I consider the "Preview" button one of my best friends .

When you're looking at a forum index page (like this one), there's a list of the threads in that forum and two buttons (above and below the list) marked "New Thread". That would've been how you started this thread (Replying to a post).

To read and reply to a particular thread on that page, you first click on the thread title and get taken to a page with just that one thread on it. The page where you're reading this reply is probably an example. If there are buttons at the top and bottom of the page, they'll say "Add Reply" instead of "New Thread". Clicking on either button should give you a text box where you can type your reply.

The "quick reply" box at the bottom of the page may already be enabled; if it is, you can just start typing in it without clicking anything first. If it's not, there should be instructions in the box for what to click to enable it.

Under every post in the thread there should be a row of buttons for (among other things) replying to that particular post. If the markings on the buttons don't mean much to you, try hovering your mouse pointer over them:

If it's your own post and you're still within the time window for editing it, there'll be an "Edit" button at the left of the row.

Next (or first, in most cases) is "Reply with Quote". That'll give you a text box with the previous post already in [quote] tags, ready to reply to.

Next is "Multi Quote this Message". More about that one here and here, when you're ready to make use of it. I often go for months without using that feature though I did use it for this reply.

Next is "Quick reply to this message", which just takes you to the Quick Reply text box at the bottom of the page (and turns on the box if it happens not to be on already).

At the far right is "Thanks" which adds your "Thank for this" below that particular post. If you thank someone for their post and later change your mind, you can use the "Remove your thanks" link that'll appear near your thank-you.

So -- were (or are) some of those actually not showing for you, or had you just not found them yet (or not figured them out) when you asked?