How do I housetrain a dog that is 15 months old?

I had a Chihuahua that was just like that. What I did was I bought a small dog crate and I put her inside, I would take her out every hour and take her outside and say "go potty" she would go and I would praise her and then put her back in the crate. Every dog is different but it only took 2 days and she never had another accident. I hope it works for you. Just remember to expect her to whine but just say no in a firm voice and don't let her out except to eat, drink and go potty.