I had the hardest time finding a verse in the Bible to show that premarital sex was a sin. My main problem was that I was looking for the wrong words. The only thing I could find was verse after verse about adultery. I was hoping to find something that defined adultery as sex outside of one's marriage, or premarital sex. I know that it is very often used this way. I was afraid I'd have to go find out what the original Hebrew meant or something. I never did find an answer to this question.

After a quick search on Google I realised the error of my ways. Here's an excellent verse that very clearly shows premarital sex is a sin:

Acts 15:20 But that we write unto them, that they abstain from pollutions of idols, and from fornication, and from things strangled, and from blood.
There are similar verses in Mark 7:21 and Acts 21:25. Fornication is defined as sex outside of marriage. With numerous references to adultery, several to fornication and the much stronger teachings of Jesus Christ about lust, I'd have to say premarital sex is definitely a sin.