First of all, I would say that the media determines just about everything we know or think we know. For example, I have no proof, outside of the media, that Barack Obama even exists or that there is a place called Afghanistan.
Second, I would say that the media affect what we expect in our personal lives. We see people living their lives in particular ways on TV programs, for example, and we base our expectations for our own lives on what we see. This goes for advertisements as well.
So the media affects us by giving us our information about the world and by changing our expectations about what our lives should be like.