The second drive needs to be completely unformatted - no file system, no partitions, no nothing. NT will create everything as an exact duplicate of the primary disk.
Highlight the primary disk, hold the Ctrl key and highlight the second, select "Fault Tolerance" and "Establish Mirror" (verbage may be different). NT will do the rest and yes, it does take a while. Your system will still be available during the mirroring process, but will be real sluggish...