How do I stop my 3 month old puppy from peeing inside?

Hello, I tell you that they gave me a puppy when she was 45 days old, today she is 3 months old and I currently have a 3-year-old dog, and I teach the new puppy not to pee inside, and I can't achieve it, my adult dog doesn't eat anything off the floor and doesn't pee at all, he's a real gentleman, but I don't understand why the puppy doesn't copy my dog's attitudes, I'm getting frustrated because I don't want to return it.