Take digital photographs of the room. Upload the photo to a color selector site and "try on" different paint colors. (See Resources.)
Choose complementary paint colors for more impact, or matching shades for a more subtle effect.
Pick a color you love. Rather than choosing a color because it's currently trendy or looks great in a model home, choose a color you already gravitate toward for inspiration.
Regard the ceiling as a fifth wall and don't assume that because it's a ceiling, it should be painted white. Consider a lighter hue of the wall color, a complementary color that matches the flooring---or paint it a darker color than the walls to make a large room appear cozier.
Purchase quarts or small testers and paint samples in the room. Live with the colors for a day or two to assess them in different lights.
Ask the paint store staff to recommend a primer. Often colors can be applied in two coats without a primer, even if they are much lighter or darker than the existing colors. If the color change requires more than two coats, prime the walls, ceiling or trim with a primer tinted close to the new color.
Paint your room. The standard steps are surface preparation, ceilings, walls, and finally trim. (See Resources.)