Well, as a Catholic, I'm not going to attempt to interpret the Bible, here's the commentary from the New American Bible:
A new creation: rabbis used this expression to describe the effect of the entrance of a proselyte or convert into Judaism or of the remission of sins on the Day of Atonement. The new order created in Christ is the new covenant.
I asked a similar question concerning St. Thomas Aquinas' Hymn concerning the what he meant by the New Rite and apparently he's talking about the New Covenant and the New Covenant is the Gospel message which is Salvation through Christ.
At a superficial level, I'd assume he was talking about Baptism. When you're baptized you're made a "new creation" in Jesus. The new creation would be the new life of the spirit dwelling within the newly minted Christian.