If the existing header can stay, just fill in the other 9 ft with studs 16" OC. Double the last one to support the door trim. Depending how the opening is trimmed, you may need to cut away some 2x6 so the siding will go on. If you can measure accurately you can build your wall laying on the ground, then raise in into position. For such a small opening, it may be easier to build it in place, toenailing the studs to the header. You might consider laying a course or two of blocks under the bottom plate, to get it, and the siding, off the ground. Look at how the rest of the garage is done.