I don't want to seem nasty, and I really don't intend this as a flame. But if you don't know how this works you need to take your bike to a shop to have the stem installed. This is for your own safety. Do it wrong and the bars can come off the bike, or twist in the steerer tube, and you can't steer, big trouble.
Please go to a shop to have the work done.
Many shops also offer a bike mechanics class in the winter, find one and sign up, then do the work your self.