First of all, think about what a tragedy is: a play in which the protagonist, usually a man of importance and outstanding personal qualities, falls to disaster through the combination of a personal failing and circumstances with which he cannot deal.
Keeping this definition in mind, most of the lines in the prologue alude to the fact that the play will be a tragedy. It talks about how civil blood will make civil hands unclean, how two star-crossed lovers will take their own lives, and how their deaths will end a long-term dispute between two respectable families.
In other words, there is foreshadowing of disaster and death just within the first fourteen lines of the play.