When is it blog plagiarism/stealing/etc?

Hi, I'm a newbie to blogher and hope you can help me with some guidelines. I understand basic plagiarism but I'm not sure about some gray areas. For instance:

If I quote a few lines of someone else's blog or online content and then link to their content, is that wrong?

Should I always contact the author/owner if I mention their product/content/article/story?

How do I get permission to use someone else's blog material? Simply by emailing them or is there some sort of protocol?

TIA!