If the dirt is fairly loose, you can use a mixture of non-abrasive, dishwashing detergent in water and a long-handled, soft, bristle brush to apply it. However, if the dirt and grime is a bit more stubborn, you'll need a Power Washer (NOT at 1200 PSI mind you.) Wet the wall, brush on the cleaner, and rinse. Start from the bottom up; NOT the top down. If you start from the TOP, by the time you get to the bottom, you'll have streaks where the cleaner 'sat' longer than it should. The Home Centers sell a siding cleaner too.