To make your own car washing detergent at home, mix 1/2 cup dishwashing liquid and 1/2 cup baking soda with 1 gallon of warm water. The baking soda helps get heavy dirt and grime off of your car, but is not abrasive enough to damage the car's paint.
Add borax to the recipe to prevent streaking and leftover residue while washing.
A more environmentally-friendly recipe is to mix 1 cup of dishwashing liquid and 1 cup of non-petroleum-based powdered laundry detergent that contains no chlorine or phosphate in 3 gallons of warm water. Vegetable-based oil soaps are also good options for the environment.