In the lower right hand corner of each reply, there's a radio button labelled QUOTE (Right next to EDIT, and below "Report this post to a moderator | IP: Logged".
Click on that button in the reply you want to quote.
If you want to put any other text in the quote use can use the vB code QUOTE at the start and /QUOTE at the end. To use vB codes just enclose the word in square brackets.