After Hecuba had a dream about her son, her and Priam sent Paris to be killed by Agelaus so Paris will not be the downfall of Troy
Agelaus couldn't bring himself to kill Paris, so he left him in the woods to perish. A bear took pity on Paris and kept him safe through the night, until Agelaus found him still alive and took this as a sign that he must live
Many years later, when Paris was a young man, he offered a golden crown to anybody who could defeat his prize bull. Ares transforms himself into a bull, won, and as promised recieved the crown. The Gods saw how fair Paris was for giving Ares the crown.
Back on Olympus, home of the gods, Zeus planned a feast but didn't invite Eris because she always caused mischief. She found out, showed up to the feast and threw the Golden Apple of Discord into the crowd. "For the fairest" was written on the apple, the only women left standing after the chaotic fight were Aphrodite, Hera and Athena
Because Zeus observed that Paris was fair and told the truth, he sent Hermes to inform Paris that he must choose who is the fairest. Hera promised to make him king of Asia and Europe, Athena promised him wisdom and to make him the greatest warrior, Aphrodite promised him the love of Helen of Sparta, the most beautiful mortal in the world. Paris chose Aphrodite
After Paris gave Aphrodite the golden apple, he raided the house of King Menelaus and took Helen who was now in love with him. When the king found out, he gathered together her former suitors who vowed to bring Helen back no matter what. This was the beginning of the Trojan War