Before you start working, you'll need to take everything off your porch. If you have any delicate ground plants nearby, be sure to cover them with protective drop cloths.
If you have a wood porch, sweep away loose debris with a broom and clean between the cracks using a slender stick or tool. Remove dirt, dust, oil and grease with a garden hose or a pressure washer. Begin at one end of the porch and continue until you have rinsed the entire surface area, removing as much dirt as possible. Spray between porch boards to eliminate any remaining debris. Let the surface dry for about 24 hours before you paint.
For a concrete surface, scrub the porch clean with a mixture of water and detergent. Rinse thoroughly to remove all of the detergent. If the porch surface is already painted, clean it with a solution of Trisodium Phosphate (TSP) and water. Follow the manufacturer's instructions when using TSP. Let your porch dry completely before moving on to paint.
Scrape off any existing paint using a scraper, pole sander or power sander—both of which will make the job a bit easier. Afterwards, mop the surface to remove dust and debris from the removal of the old paint.
TSP can be very corrosive. Always remember to wear rubber gloves and eye protection when working with chemicals like TSP, as well as a pressure washer, sander and primer and paint. Consider wearing kneepads if you need to work on your knees.