Quoting somebody else's text is really very easy. Have a look at this screenshot I have taken. You will see I have encircled the "quote" button that you need to click if you want to quote some text. Once you click it, just type your text underneath the other person's text.
Sometimes you may only want to quote a small portion of what somebody has written, not the whole thread. It's quite simple, just delete any text that you don't want to be quoted. Always make sure that the quote tags are in place, they will be either end of the quoted text, you can even add them manually if you want to have various quotations with your own text underneath. Basically you put your text in between the quote tags.
