This is a great question. One of the most interesting points about Homer's Iliad is how infrequent Helen is mentioned. In fact, Helen appears only six times in the epic, and four of these times are in the third book. The other two appearances are in book 6 and 24.
As the Iliad starts in the ninth year of the Trojan war, it really does not show how Paris met Helen. The abduction of Helen is intimated. The best that we can say is that Paris "won" Helen, who was considered the most beautiful woman in the world as a reward for settling a contest among three goddesses. The dispute was who was the more beautiful among Hera, Athena, or Aphrodite.
Each goddess promises something different. Hera promised rule of Asia. Athena promised prowess and success in battle, and Aphrodite promised the fairest woman in the world. She left out the detail that she was married. So, Paris goes to visit Menelaus and abducts Helen. All of this is intimated in book 24.