The easiest way is to just click the "Quote" button at the top of the post you're replying to, rather than clicking "Post Reply". This copies the post text to the top of your post and surrounds it with the special quote BBCode tags.
Short of that, you can copy and paste the particular part of the post you'd like to quote, and place [quote] and [/quote] tags around it yourself. If you want to include a name, you can do [quote="Name"] and [/quote] instead.
See the following:
In general, though, you should only quote if the post you're replying to is far enough up the thread so people may be confused who you're talking to, or if the post you're replying to is so large that you want to show which part you're specifically replying to.