If we consider Genesis 5, we'll see the omission of Cain and Abel. It's at 130 years that Adam has a son named Seth. (v. 3) Either information about other offspring has been omitted, or prior to the birth of Seth, there were only Adam, Eve, Cain, and Abel. Omission of other offspring is possible as this does occur in the Bible. If this is the case, then there were probably other offspring from Adam and Eve, whom Cain would have feared for his life.
In Genesis 4:16, it says Cain settled in the land of Nod. This is wordplay, since "Nod" means "wandering" (נוֹד), which Cain was punished to be a wanderer (נוּד). Whether or not the land of Nod was already a settlement, I don't know. Yet, in v. 17, Cain sleeps with an unnamed woman. The pseudepigrapha, the Book of Jubilees (or Lesser Genesis, an ironic name), Adam and Eve also have a daughter named 'Âwân (אָוֶן; "iniquity"), who ends up being Cain's wife.
And Cain took 'Âwân his sister to be his wife and she bare him Enoch
at the close of the fourth jubilee. (Jubilees 4:10)