Look to the top right of this post.
You will see two buttons labelled "Reply" and "Quote".
If you hit the reply button, you will create a new post starting with a blank page. However, if you hit the "quote" button, it will insert the text of this post enclosed within a "quote block" - ie. enclosed between the {quote} and {/quote} (substitute "[" and "]" for "{" and "}" ) commands, into your new post, after which you can compose your reply.
If you only wish to quote part of a previous post, then all you have to do is delete the bits you don't want.
See
This link for a list of basic BBcode instructions (note that some may not be implemented here)