Before you start painting with your new color, you'll need to clean the surface so that paint will better adhere to the wall. We've got something that can help you clean those walls quickly, without too much effort. We advise taking a sponge and trisodium phosphate cleaner (or TSP substitute) to wash off all that extra dirt. For something even less expensive, buy TSP concentrate and mix with water - you can get a 16-oz box for about $5.
TSP can be used on all of your woodwork. It can even be used to clean grease from kitchen walls, remove hairspray in bathrooms, an take off fingerprints around light switches. To avoid TSP solution making contact with your skin, wear rubber gloves and turn up the cuffs to catch the drips.
For more painting how-tos check out Family Handyman - Painting