Carpeting is the best way to reduce noise levels, as it absorbs the sound waves rather than reflecting them. Using a rug can help with the sound if you can't change the floors.
You can also add soft furniture to the room. Along with carpeting, a soft couch, armchair, or any other soft furniture can achieve this sound-absorbing effect.
As windows and doors are often much thinner than walls, you may want to use weather stripping to seal any cracks to help prevent sound from entering or leaving.
You can also add an extra drywall layer to the walls and ceiling for added dampening power. There are specialized caulk and drywall boards specifically for soundproofing.
Adding a floor-to-ceiling bookshelf can provide sound-deadening mass to your room. Consider sound absorption paneling or using thick blankets on the walls for more sound dampening.