Just go to any part store (usually) and get Duplicolor Wheel Paint, they make spray paint specifically for painting hub caps and wheels. It has directions on the can as well. You can also visit
http://www.dupli-color.com for more information.
I would buy 2 cans, but 1 can might do you. Also buy some fine sandpaper to sand down any imperfections you have from the scratches and a fine sand over the existing hub cap surface. After clean off any dust from the sanding, and follow the directions for spraying the paint.
Good luck!