According to Genesis, God created Adam and Eve.
They disobeyed God and as a result, they were driven out from heaven.
They had two sons, Cain and Abel.
Cain killed Abel and God punishes Cain.
After that, it says that Cain married.
But whom did he marry?
And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch.
Genesis 4:17 (KJV)