How to quote another post?

I use my computer everyday. But I really don't know just what I'm doing. I noticed that when someone posts something and another person want to respond and (quote) someone often time the whole post is highlighted in a light blue color. But sometimes someone just wants to (quote) just a portion of the question/statement.

So now the real question HOW DO YOU (QUOTE) JUST A PORTION OF SOMEONES POST? Thanks,