Alright, Welcome to Forums 101.
First, the "Quote" button next to the avatar allows you to reply to the poster you wish to reply to.
Next, the "Full Editor & Preview" button allows you to do more than what you can with the standard quick reply. These things include adding images, urls, multiquoting, etc.
To multiquote, you need to be in the Full Editor and scroll down to the bottom of the page where you will see the posts on that thread. Click the "Quote" button on each message you want to reply to. This will add BBCode to your post, and will look like this:
Just press "enter" after the BBCode, and write your reply.
Also, when you get enough posts you can enter the Chat area (although, you might not want to given how random it can be
). Hope that helped some. Any other questions?