Paris held a bull fight and said "whoever wins will be rewarded with a golden crown". Naturally Ares the god of war accepted that challenge and naturally won. Paris gave Ares the crown and he became known as a man of his word.
meanwhile, Zeus was holding a wedding feast and he invited everyone except Eris who was a goddess of misery. When Eris found out she took a golden apple that was for the "fairest" of them all, and threw it in the crowd.
A fight broke out and only Athena, Aphrodite, and Hera where left, who where fighting over the apple. Zeus immediately knew who to make the judgement.
Zeus asked Paris to make the judgement and Paris accepted. All the goddesses made promises. Only Aphrodite's offer did he want. She offered him the Love of Helen of Troy who was considered the most beautiful of them all. He gave Aphrodite the apple
And so Paris went to Helen who immediately fell in love with him and they both went off together away. Only later did the king realize what had happened.