Washing the exterior of your home before painting will not only get the outside of your house clean and provide a dirt-free foundation for the new coating, but it will also help you find surface flaws that have to be dealt with.
Depending on just how dirty the outside of your house is and on the house's size, there are two ways to approach this job. If you live in an average-size house, use a garden hose with a carwash brush attachment to bathe the big areas. For caked-on dirt, use a scrub brush or a sponge and a pail of warm water with a good, strong household detergent in it. Work from the top down, and rinse all areas where you scrubbed with water.
For bigger houses or for faster work on smaller ones, rent a high-pressure spray cleaner. This device attaches to your home's water-supply system and puts out a jet of water at a pressure of about 600 pounds per square inch. It is equipped with a handheld wand tipped with a trigger-activated nozzle. The pressure is high enough to dislodge not only stubborn dirt, mildew, stains, and dried-on sea-spray salt, it's enough to remove peeling paint. In fact, if the jet nozzle is held too close to the surface it can even peel off perfectly sound paint, split open shingles, and drill a hole in siding. So follow the manufacturer's directions and wear goggles and protective clothing.
You can use the spray cleaner while working from a ladder -- although scaffolding is better -- but practice at ground level first; the force of the spray against the house could knock you off a ladder if you're not careful. Some of these machines come with separate containers you can fill with cleaning solutions or anti-mildew solutions. Sprayers are so powerful that ordinarily you probably won't need to use a cleaning solution; if you do, remember to rinse the surface with clean water afterward.