The ten commandments are part of God's moral law, which is eternal. They apply just as much to us today, as they did to people under the OT. In fact, 9 of them are restated over and over in the NT, especially in the epistles. The one commandment that is not restated is the sabbath - because the NT teaches that was a picture of Christ, and our rest is fulfilled in Christ.

Galatians 3:24 Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith.

James 2:10 For whosoever shall keep the whole law, and yet offend in one point, he is guilty of all.

The Law was given to show we were sinners in need of a Saviour - if it was wrong for me to break the commandments before I was saved and therefore was in need of a Saviour - it is still wrong today to break those same commandments.

What the NT teaches is that we are not obligated to fulfill the Law for salvation, and if we are saved, we are not under the curse of the Law. The NT does not teach that none of the Law still applies - yes, there are parts that the NT teaches are done away with (ceremonial laws, the OT priesthood, the sacrifices, the sabbaths and holydays), but it does not teach that the moral law is done away with.