Cain finds a wife. In one of the apocryphal books her name is mentioned: Awan (Book of Jubilees 4:9). It must have been one of his sisters. Such a marriage in our day comes under the judgment of incest. Later in Moses’ laws, sexual intercourse with close relatives is prohibited as incest. It also leads to a lot of misery because offspring that results from this are often handicapped. That would not have been the case in this early part of human history, perhaps because human genetics had not yet degenerated as much as they are now (from the perfect creation of the first humans).
17 Cain knew his wife, and she conceived and bore Enoch. When he built a city, he called the name of the city after the name of his son, Enoch.