Unity Networking, where do I start?

Hey, I have made a game with no networking which is a simple 2D game. The game at the moment has only 2 settings, normal and hard mode where you play through the game collecting items and trying not to die. What I want to do is add a multiplayer option for the player so they can play against friends or random opponents, I don't want the players to be able to see each other but I do want a stat tracker so the player can glance at the other players progress whilst playing, the winner would be whoever survives the longest or reaches a target of items collected. I have never used networking before so my question is where do I get started with making this? Do I need to download any plug-ins or is it already pre-installed in unity 5?