How Do I Know What I Know??

This question is extremely hard to answer. When I think about it one can not be completely certain about something. But mainly what I know, is given to me by people or many sources for information such as movies, books, the news, etc. I either learn it, read it, or have experienced it. I use what I have learned to use in my own life. But when it really comes down to it, how do I really know that this is something I am sure about?? It is hard to know for sure. I think that the lack of certainty leads to the many questions that comes with living life. Like why do I do what I do? Plenty of questions arise all of the time, yet the answer is never really satisfying.  Sometimes when it comes to "knowing" what I know, it mostly comes from intuition, or a feeling that I am certain of something. But feeling it doesn't necessarily mean  that it is right. So does that mean knowing  isn't always right? So if it isn't right, then what is the  point of knowing something??