If you wish to quote a specific post, at the top right corner of each post you have two buttons, "Reply" and "Quote" If you press "Quote", the contents of that post will be copied to your reply. If you wish to address a specific part of that post, you can simply remove the non-relevant parts. This method will attribute the quote to the original author of the post.
If you wish to quote an external source, such as text copied from another web page, which is not attributed to a particular forum member, you can use the [ quote] and [ /quote] (remove the space following the leading bracket "[" ) tags to enclose your quoted text.