Get a local architect who works in the specific jurisdiction you are in to draft you a set of plans. They should come back with the P.E. stamp, and all the calculations you need for the work proposed. You're going to be doing foundation work, so you probably will need soil bores to determine the density of the ground on-which you're putting it, which will tell the structural engineer how to design the footers and slab.
When you pick-out the doors and windows that you want to use, and give the manufacturer's spec sheets to the architect, he/she will incorporate them into the plans for presentation to the local building dept. Using a local design professional known to the local building dept will help you to get your plans approved quickly.
I'm assuming you want to do a frame building. Consider some sort of roof structure that will give you a second story, or usable attic space, or sufficient height with enough room to install a 2 or 4 post auto lift, if that's your intent to use the garage addition for vehicle work. Also, try to design-in some sort of an I-beam that will allow you to have a trolley lift for pulling engines, or for removing heavy loads from the back of your pickup, and allow you to drop them inside, easily.
Hardy-plank, or stucco, or vinyl should give you long service on the outside walls. Also, don't forget to insulate the walls well, to make it easier to heat, for more-comfortable use. Metal wainscotting, perhaps corrugated metal for the first 4' inside wall height , will help to prevent you caving-in the gypsum board from your inside work. If you're planning on doing welding or grinding in a specific area, constantly, you might want to run it higher there.
You can never have too-many lights, and too-many outlets. A sub-panel will probably be required, and don't forget to install both 110 and 220 conveniently, for your intended uses.
