Simplified: You will have to reframe the upper portion of your studs where they will be cut off in order to accommodate the door and frame. That means taking off the wallboard or siding or other wall material at least six inches around the outer dimensions of the new door and its frame. Then, you do some measurements. You will need to brace up the framing over the door by use of a header. That supports both the joists above the frame and gives you something to attach the top of the frame into. You remove the studs that will be in the way of the door. You add some studding where necessary to accommodate the uprights of the door frame. Make sure there is solid structure on the hinge side of the door, because that is where the weight of the door is going to be supported. Once you have an opening that will accommodate the door frames, which are usually a unit that comes together with the door, you slide the door and frame into place. You then attempt with shims to make the door as perpendicular or "square" as possible, driving long screws through the frame and shims on the hinge side, into the support studs. Your frame should include some trim around what will be the outside of the door. That trim will match up with your outside wall surface and help hold the door true to the wall itself.
You then use interior framing trim to cover the door's frame and the hole running along side it, and to extend past and onto your wall.
You will need: Door with frame, hopefully one piece type construction. Shims. Some 2X4's for headers and sisters (the upright supports for the frame), nails, door hardware in the form of locks and handles, interior trim along with whatever stain or paint you want for it, finishing nails to attach the trim pieces. Equipment includes saws, a carpenter square, hammer, screwdrivers, a drill and bits so you don't split out the wood, measuring tapes, and other carpentry tools.
Lots of patience and some thought ahead of time especially about where you are going to knock your hole in the wall. Make sure you are not interfering with any electrical lines. If so, reroute them.
Actually, for a novice, it is a challenging project, since you are going to knock the hole in the wall. That does produce some thorny problems, in that your headers and sisters must be fairly precise to asure the frame fits in securely. It is okay to have 1/4 to 1/2" around the framing to the edges of the wall. You might consider expanding foam insulation for that, or you can stuff fiberglass insulation around it for weathersealing.
I would find a friend who has done something like this before and ask for help. With winter coming on, an opening in your wall even from the garage to the outside could be there a long time while you try to figure out your next steps. It would be better to learn from someone who has done it before, than to try to complete it without ever having had the experience. Next time, you will have a much better idea of how hard it is to do what you want if you don't already have some experience.