I'm going to go ahead and take a leap of faith here! I can't tell, from the OP's writing, exactly how this house/garage is set up, so I'm going to go from experience.
We did this exact thing at a friend's house, where the garage was built after the house. From the outside it looked like an attached garage, but was actually just built on. No door between garage and house.
On the wall between the garage and house, what is on the "garage side" of that wall? Siding? Drywall?
If it's drywall, I'd start there. Determine where you want this door to go into your house. Then cut out your drywall - on the garage side - to see what's in the wall. Remove a piece that's considerably bigger than where the door will go. Don't use a drywall saw and just go crazy. Use a good utility knife and cut down the center of of the studs, so that you don't run the risk of cutting wires that might be in there. Go ahead and cut all the way to the ceiling, because you're going to be going up there anyway.
If there is an electrical wire running through the wall, you can re-route it up over the door plate/header.
If it was my project, I'd definitely purchase a pre-hung door rather than trying to hang it myself.
Here is a website that lays out some very simple, basic information about this. The great news is that it really isn't as hard as you'd think it might be.
How to Frame a Door | DoItYourself.com