well, if you want to reply quickly, at the bottom of each post there are a few little buttons, "Quote, Quote+, reply, and Thanks." if you want to quote the person above you or a person anywhere on the page, click one of those first two buttons (quote is for text only, quote + is for text and pictures, smilies, etc).
Reply is for straight replies without quotes.
Thanks is for thanking people
If you want to add smilies or other more involved things to your reply, click "Post reply" right below the last post in a thread. The post reply button is on the far left and it's in red and purple.
I hope I said all of that clearly enough and I hope you find everything ok!