you have several options for a finished look. You can do as Daryl did and mount a Bi Fold pin at the top and bottom of the door, you can install a bi fold door in the opening , you can get a sectional door for the opening (they look like curtains) or use a piano hinge (this runs along the length of the door and can be surface mounted to the face or the edge).
You're best option is just to simply buy a prehung door to fit right into the opening, as everyone else has said. its the simpliest solution.: