To quote someone when you're writing a comment, press the indent button (it's next to the link button). To stop quoting, press the outdent button (to the left of it).

However, try and use the "reply" button as much as possible. It allows you to reply to someone specifically, without clogging your post up with large quote boxes.

It also gives the thread some structure - with the reply button, you can have many mini-conversations, within the one thread, which makes the entire thread much easier to follow.

If you have turned off the rich text editor in your account settings, you can use the blockquote tag.