The ideal time to begin training is when you bring your new puppy home. Puppies should be taught to walk on a loose lead, come when called and stay. Always use praise and encouragement when training your puppy. By starting training early you avoid developing any bad habits. Newfoundlands should definitely receive early obedience training.
Obedience training is the prerequisite to water, draft work and carting.
If you intend to water train, it is advisable to introduce the puppy to water by the age of four months but do so with care and consideration. Puppies can generally be housebroken within a few weeks of bringing them home. The key is being consistent and watchful. Remember though that for a few more months accidents can happen. This is not from disobedience but because young dogs do not have the muscles to wait.