Roofing and Siding: Nail C-D exterior grade plywood to the rafters, letting at least an 8" overhang on either gable end. If you wish, nail 1 x 6 fascia to the rafter and gable ends to give the roof a finished look. Place drip edge around the perimeter of the roof, then cover the roof with felt and the roofing material of your choice.
Install the windows and doors, then nail 1 x 4 corner strips in place. Working from the floor level up, attach siding to the frame. The siding pieces should butt in between the corner strips, or butt up to the window and door frames. When you work up to the gable, cut the ends of the siding at a 20o angle, so that it will fit flush against the roof.

Finishing: If you want a rustic look, coat all exposed wood with a preservative. The wood will then weather a natural gray and will need little additional upkeep. For a more "Finished" look, prime the raw wood and paint or stain it to suit your tastes.

Mechanicals: As a workshop, you'll need electricity, heating/air-conditioning and (probably) running water. For electricity and water, you'll need the services of a licensed electrician and plumber. For heating and air-conditioning, you might consider a "through-the-wall" unit that provides both. Ask your electrician for recommendations. Another good recommendation would be to install a dehumidifier to help you control the shop humidity and eliminate any potential rust problems.

Variations: If you like, you could alter the size just a bit to accommodate a roll-up garage door at one end that will allow you to "work in the open air", if you like…plus make it easier for you to bring in materials and remove finished projects.
Another option would be to put up sheathing, then use aluminum or vinyl siding to cover the exterior, rather than wooden siding (which will require much more maintenance).