The Prologue says that the only thing that will end their parents' rage is the deaths of the two kids. It tells us that the families have been fighting for a long time, but it does not tell us why.
Romeo and Juliet meet at a ball that is given by her family. He does not want to go, but his friends kind of pester him into it. Just before he goes in, he tells us that he thinks that something bad will happen there -- something that is going to lead to his death.