According to what Gatsby tells Nick, he met Daisy during World War I. He was stationed at a camp near to Louisville, Kentucky. That was the city where Daisy (and Jordan Baker) lived. While he was there, he and many other of the officers at the camp would come into town. Many of them were very interested in courting Daisy.
Daisy and Gatsby were really quite interested in one another. But she ended up rejecting him because of his lack of money. This was why he became so obsessed with making money.