The Capulet family and the Montegue family have had an ongoing arguement... Tybalt Capulet and Benvolio Monague bump into eachother one day and... it doesn't go down well.
One night, Lord Capulet throws a party for all the Capulets and friends of them. Mercutio, a rather loopy man brings Romeo Montague to the party where he meets Juliet Capulet.
Romeo insisted that he is in love, that it is love at first sight. He takes Juliet outside where they kiss. Meanwhile, Tybalt Capulet (Juliet's Coisin) looks around for Romeo so he can kill him.
After the party, Tybalt wants revenge on Romeo for going to the party so he finds Mercutio, Benvolio and Romeo on the beach and challenges Romeo to a duel. Romeo refuses so Mercutio stands up instead and ends up getting defeated.
Out of vengeance for Mercutio's death, Romeo lost his temper and stabbed Tybalt. This made Juliet confused and upset but decided that it was for a good cause.
Even though Romeo just killed Juliet's cousin and that their families hate eachother, they were still very much in love and so they decided to plan to run away (Ignoring that Paris was due to marry Juliet) and secretly get married.