Before prepping a room for painting, protect the furniture and flooring against splattering paint or accidents.
Dust and clean the walls. For most surfaces, use a towel or a vacuum cleaner. When painting a bathroom or kitchen, wash the walls with a solution of approximately three teaspoons of laundry detergent to one gallon of water.
Scrape any cracked or flaking paint with a paint scraper. For other small imperfections on the wall such as plaster bumps, smooth them away with sandpaper. Use muscle power with a piece of sandpaper stapled to a sanding block, or use an electric sander.
Materials to have on hand when getting ready to paint: primer, paint-can opener, stir sticks, roller covers (close nap on a rolling cage, large nap without rolling cage), paintbrushes (one for latex paint and one for oil-based paint), paint tray, gloves, drop cloth and an extension pole for the roller.
A high-quality primer will help to hide any small imperfections on walls. Use a good water-based primer on new drywall. Choose an oil-based primer for walls that have heavy stains from water or smoke damage, or if the wall has paneling.
Before painting, protect wall sconces and wall trim with painters tape. Start in the corners of the wall and up near the ceiling, "cutting in" the paint with an angle brush.
With a small brush, apply the paint around trim and in the corners of walls where your larger brush can't reach.
A great tip is to do a "W" pattern to paint walls. Start in the corner of a wall and roll on a 3' by 3' "W"; then fill it in without lifting the roller. Repeat until the section is finished. This helps hide seams and any places where the roller has been lifted and put back on the wall again.