Gen chapter 4 vs: 13 Cain said to the LORD, "My punishment is more than I can bear. 14 Today you are driving me from the land, and I will be hidden from your presence; I will be a restless wanderer on the earth,
and whoever finds me will kill me."
15 But the LORD said to him, "Not so [e] ;
if anyone kills Cain, he will suffer vengeance seven times over." Then the LORD put a mark on Cain so that
no one who found him would kill him. 16
So Cain went out from the LORD's presence and lived in the land of Nod, [f] east of Eden.
17
Cain lay with his wife, and she became pregnant and gave birth to Enoch. Cain was then building a city, and he named it after his son Enoch. 18 To Enoch was born Irad, and Irad was the father of Mehujael, and Mehujael was the father of Methushael, and Methushael was the father of Lamech.
Notice this is before Seth is born, Seth is the third son and before any daughters are born to Adam and Eve.
He is worried that people will kill him, in the land of nod he meets and marry's his wife.
Its the 4th chapter of the first book, my gosh its not that hard to get that far