This is one of the most common questions among Christians. Ken Ham answer very well to this question.
In the Bible, in the New Testament Paul tells us in 1 Cor 15 verse 45 that Adam was the first man. Now in the first book in the Bible Genesis chapter 3 verse 20 we read that Eve was to be the mother of all the living. So according to the Bible all human beings alive today, all human beings ever lived are descendants of one man and one woman, Adam and Eve. There are only one race of human beings.
Skeptics of the Bible often asks if God only made Adam and Eve and they had three sons Cain, Abel, and Set as recorded in Genesis, then “where did Cain get his wife?” Well, Genesis chapter 5 verse 4 records that Adam had other sons and daughters. In facts jewish tradition is ahead of fifteen children, that means original brothers would had to marriage sisters. Now you may say “wait a minute brothers aren’t allowed to marriage sisters even the Bible says that!”
Well Abraham was married to his half sister and that wasn’t the problem, actually it wasn`t to the time of Moses that God said in Leviticus no longer can close relatives marry. So why is it that close relative could marry originally but close relative shouldn’t marry today? When Adam was created his gene was perfect! Eves genes was perfect! But, Adam sinned and as a result of sin God no longer holds everything together perfectly. So now everything runs down, the further away relationship you are from each other the more likely it is that you have different mistake. And so when you get married and have children you get a good gene from one partner with a bad gene from the other, and the good gene tense to master the bad gene so it doesn’t show up in the next generation to a great extent in the sense of a deformity or some major problem. But the further back you go in history the lesser problem that is. You see, originally Adam and Eve had no mistake, their children would have relatively few. So, provided was one man for one woman which the doctrine of marriage is all about. There is no problem with close relatives marriage original. So, you see it is easy to understand where did Cain get his wife. Cains wife was one of his relatives, a close relative at that time but one of his relative just as we marrige our relatives today.