Knowing Your Enemy

When the Bible talks about the Spiritual forces of Evil, when it talks about Satan, Demons and the demonic … what is it talking about?? 

Is this something we should fear… and be afraid of… What is the agenda of the demonic? Like what are the Spiritual forces of Evil trying to do?

And What is the Christian to do when it comes to the demonic … is this a threat to me? What do I do to protect myself … is this something I need to do to actively fight?