Here's how it happens:
1. Nick is invited to a party at Gatsby's mansion. He attends with his friend, Tom Buchanan, and his wife, Daisy.
2. Gatsby's presence is felt before he's actually seen. The guests gossip about him, speculating about his past and his wealth.
3. Nick meets Gatsby at a party. The two men are introduced by Jordan Baker, a professional golfer.
4. Gatsby appears as a mysterious and enigmatic figure. He is described as "tall, with a broad chest and a firm chin" and with "a touch of paternalism in his voice."
5. He seems interested in Nick. Gatsby tells Nick that he's "just a man who likes to have a good time." He also reveals that he is a self-made man, but refuses to elaborate on his past.
This initial introduction is crucial in establishing Gatsby as a mysterious and intriguing figure. The novel is structured around Nick's observations and interpretations of Gatsby, and this first encounter sets the stage for the unfolding of their complex relationship.