The *rough opening is the width and height between the studs side to side, and between the sub floor and the bottom of the header for the height.
The door swing will either be *right hand or *left hand. To determine this you place yourself under the door header with one leg in one room and the other leg in the adjoining room and your back up against the hinge side of the new door. If you want the door to swing on the left you order a *left hand door, just the opposite for a right hand door.
The best way to order these interior doors is what we call *pre-hung. They are already mounted on a frame and the hole is drilled for the knob. Make sure to check your wall thickness so you can order the correct width door jamb. Standard jamb size is 4+9/16" wide.
You can order these doors in natural unfinished wood such as mahogany,birch, which are veneers. Or you can order them primed for painting or pre-finished in a stained color. Lots of options.
Check light switches if they are already in place, as you don't want to hinge a door in such a way that the light switch has the door swinging over it.
Typically, doors always swing into a room from a hallway or larger open area.
Carpenters refer to door sizes in feet and inches. So a three foot wide door , we call it a 3'0" (we say a three / 0 ) three oh. A 32" door we call a 2/8 and so on.
Your rough opening is always 2" larger than the door size width and 82+1/2" on the height. If you already have a finished floor you can use 82" for the height.
Now remember bi-fold doors/ by-pass doors / and pocket doors are different on rough opening and finished openings - check with supplier on the rough openings for these doors.
I also prefer solid jambs on pre-hung doors as opposed to *split jambs. Split jambs will adjust to fit odd wall thicknesses, but they are flimsy and cheap. Order solid jambs on your prehung doors.
Good Luck