How do I hang a mirror? Details below....?

Sorry if the question seems basic, but I want to do it right and have never done it before. I also don't want to damage the wall. The mirror will go above the fireplace (I plan to rest the bottom edge on the mantle). The wall is plaster-- I am afraid that if I put a nail in, there might be a long crack in the plaster. There is a wire on the back of the mirror (to facilitate hanging the mirror). Any advice, even small details will be very much appreciated. Thanks.