This is a particularly important issue to be aware of in today's academic climate, where internet usage has made plagiarism both so easy and so pervasive. It is therefore vital that every student is very aware of what needs to be cited and what does not.
The rule is that any piece of information--statistic, quote, opinion--that comes from another source is something that needs to be cited. So, for example, a statistic that you found from a government paper needs to be referenced, just as the opinion of an academic that you interviewed needs to be cited. Of course, if you are lifting a direct quote from an interview or another paper, you need to show that to be a direct quote by giving a direct citation after it and using speech marks to indicate you have taken in verbatim.
Therefore, it follows that what does not need to be quoted are your own opinions and accepted, commonplace facts that are not the opinions of others. I have included a link below to help you further in this matter.