To repeat so that you know I am on the same page as your problem: You apparently bought a door frame "Kit". This door kit includes the door with the side jamb or the area with the hinges attached and the side for the lock attached.
The actual door will be the same size, but you have to remove the old jamb entirely and create what we call the "rough opening". You take the molding off carefully on both sides then pry off the old jambs all the way around. When this is done, you carefully and skillfully plumb up the new door in the opening and square it off, then use shims to maintain that squarness then fasten it to the wall studs. Then you attach the molding back and install the locks and lock hardware. Then caulk and prime and paint.
from reading your post and if my accessment is correct, I would strongly recommend that you contact a good handyman to do this job. Installing the door is easy, its the shimming, squaring, and maintaining square that is the tough part.