As a former groomer, I can say that Cavaliers are easy to groom and care for. They have a lovely soft double coat (meaning the shinier top coat and a downy undercoat), and a brush and comb out once or twice a week should keep them unmatted. If you have more than one Cavalier, they tend to pull each other around by the ear when they play, and they can develop tangles similar to dread locks. These knots can be picked apart with your fingers, then brushed and combed. Cavaliers should be bathed regularly with a good quality shampoo and conditioner.
Like most dog breeds, Cavaliers King Charles Spaniels shed. The more frequently they are combed, the less loose hair will be on your floor.
Their nails need regular clipping. If you are nervous about clipping them, a groomer can do that for you. (You will receive detailed grooming instructions with your puppy.)