A newborn puppy is a virtual muscle, bone, hair and teeth-making machine. One of his main jobs in life (besides playing) is eating. He has to consume a lot of nutrition if he's going to grow from infancy to adulthood in just 12 months (a large breed puppy may take as many as 24 months to reach full development). Naturally, the better his quality of nutrition, the better the foundation you'll be laying for his future.
Mother's milk is the ideal first food. Although he won't be ready for weaning until six to eight weeks, he will start to nibble solid foods at three to four weeks. (Truth is, he'll mostly romp in and out of the food bowl, licking the food from his paws.)
Regardless, this is a good time to offer small portions of a high-quality puppy food like Purina ONE Healthy Puppy Formula, moistened according to package directions. Unlike adult dog food, it will provide extra calories and nutrients your puppy needs for bursts of energy and rapid growth.