DAY ONE! Everything you do with this pup is training. If you play "fetch the sock in the hallway", it is actually teaching the pup that you like it when he retreives. When you get on to him for something it teaches him that you don't like that. And when you let him tear things up, you teach him that the boss man likes it when I tear things up. Like the other guys said, take it slow, have fun, and don't over do anything. Don't play fetch until he gets bored. 3 or 4 hallway/backyard retreives a night is fun for all and won't overdo it. I like the hallway because after you throw the sock and he goes and gets it he has no where to go except to bring it back to you. If somehow he doesn't bring it back to you, Game is over. Stop playing with him. He will learn that in order to have fun with you he has to play by YOUR rules.
Little pups are my favorite thing in the world.