The Bible doesn't say where Adam and Eve's first two sons -- Cain and Abel -- got their wives, although it does tell us that Cain and his wife had at least one child (Enoch). The usual assumption is that Cain and Abel married their sisters. (Later this was forbidden by the Old Testament, but was necessary at the beginning of the human race.)
Don't be sidetracked by questions like this, however -- because the story of Cain and Abel is an important one, and has many lessons to teach us. (You can read it in the fourth chapter of Genesis.) Cain, the Bible tells us, became angry at his brother because God had accepted Abel's sacrifice but rejected Cain's sacrifice, because Cain was selfish and his heart wasn't right before God.
Over time Cain's anger grew, and eventually he killed his brother -- thus committing the first murder. God asked him where his brother was, and his reply has echoed across the centuries: "I don't know.... Am I my brother's keeper?" (Genesis 4:9). Heartless and totally wrapped up in himself, Cain became the ancestor of all those through the ages who have lived by violence instead of brotherly love.
The story of Cain and Abel, however, points to an even greater truth -- and that is the reality of human sin. Like a deadly cancer, sin invaded the human race with Adam and Eve, and brought evil and conflict in its wake. But Christ came to reverse all that -- and it begins in our lives right now as we yield ourselves to Him. Has this happened in your life?