Our story starts here, at the reception of Peleus and Thetis' wedding. All of the goddesses on Mt. Olympus had been invited to the wedding. Except for one, Eris the goddess of disagreement.
Eris was so furious that she was not invited to the wedding, that she showed up unexpectedly anyways. She then threw a golden apple on a table which had the words "to the fairest" inscribed on the apple's skin.
The three goddesses, Athena, Aphrodite, and Hera, all thought they deserved this prized gift.
Not wanting anything to go wrong, Zeus throws the golden apple out of Olympus and into Troy. Paris, Prince of Troy, picks up the apple wandering by and the three goddesses appear for Paris to decide who is the fairest.
Paris decides Aphrodite is the fairest in exchange for Helen's, the most beautiful woman's, love.
Even though Helen was already married to King Menelaus, Aphrodite's love spell made her fall for Paris. Helen and Paris run away to Troy where they hope to live happily ever after. This starts the Trojan War.