This is potentially not a DIY project, unless you have some knowledge of framing and setting door properly. Basically, you need to remove the garage door and all its parts including the finishing materials around the opening. This will expose the framing of the garage wall. Build out new framing in the open space leaving the necessary R.O. (Rough Opening) for the doors. The RO will depend in the doors you select. Install the door into the opening and then finish off the rest of the newly exposed framing with drywall on the inside and whatever finish material you currently have on the outside. You will also need to install insulation the the new section of framing prior to closing it up. Trim out the door on both sides and that's it.

Hire a contractor!!