How to do a car wash?

I am 12 and i want to get a bit of money. So I want to do a car wash. How should I accomplish this? Should I stand in my driveway and hold up a sign that says "Car Wash!! $7" (or whatever price) so that people driving by can pull in? Or should I hang up signs around the neighborhood and go to other peoples houses and wash their cars at their house? Thanks!!