Doggie Help !! Pleasee?

I just got a long haired chihuahua and hes about 9 weeks old hes name is Milo but i read online that they can be aggressive and mean but i haven't seen that with Milo hes hasn't barked once since i got him and he knows not to pee in the house but i read that they can be protective and can snap back at any time and what can i do to make sure he doesn't turn out like that and what can i do to make sure hes always good.