The problem of who Cain married is only a problem if you don't know what the Bible says. If you have only a superficial knowledge of the Bible you may think the Bible says that Adam and Eve, the first man and woman, had only two sons — Cain and Abel. 
But the book of Genesis tells us that, besides Cain and Abel, Adam and Eve had other sons and daughters (Genesis 5:4). Jewish tradition teaches they had 33 sons and 23 daughters. (Remember that Adam lived 930 years, so he and Eve had plenty of time to have 56 children!)

 

Getting the population started
When God created Adam and Eve, He blessed them and said, “Be fruitful, and multiply” (Genesis 1:28). To get the population started and to multiply, at least one of Adam and Eve's sons had to marry one of their daughters.

It is likely that in the first generation all marriages were between brother and sister. Living so close to the time of the perfect creation, no genetic problems would yet have developed, so no harm would have come from such close marriages. God had no need yet to institute laws against close marriages. 
By the time of Moses, nearly 2600 years later, many biological problems would likely have been accumulating, which made marriage between close relatives dangerous because of the risk of deformities in the children. So laws were given against marriage between very close relatives (Leviticus 20:11-20) and are still in place today. 
We are not told how old Cain was when he went out to build his city (Genesis 4:17). But Bible commentators have estimated that at least 120,000 people could have been alive on earth within only a few hundred years. So Cain probably had quite a lot of women to choose from for a wife. 
If he married early in his life, his wife would have been his sister. If he married late, it could have been one of his countless relatives.