How do i get my dog to stop barking?

I have a 5 month old puppy and he enjoys barking up a storm when a cat walks by the window or he hears a noise or something. He's really small so he'll only really do this if he has something to hide behind, but he hides behind me and our husky a lot so it's a nusence at 3 in the morning when he wont shut up. He wont do it outside only while he's inside.