are chihuahuas hard to potty train?

They're not harder to train, they're just harder to supervise, so starting out right is essential. Get a crate (the smallest they make is usually too large) and block off a section if need be. Don't make the mistake of letting it have the run of the house in the beginning, or it will be sneaking under furniture to eliminate once it gets caught in the act. Keep it confined in the beginning to one small room (like a kitchen with a tiled floor) where the crate is located and make sure you stay in there with it when it's not crated. There are tons of sites that explain crate training in detail. It's a good idea to get a thorough understanding of how it works before bringing your dog/puppy home

Oh, and the crate isn't just used for when you're not at home. The dog should be kept in it any time you can't supervise 100%

Any dog can learn to be potty trained it takes a time, everytime they pee outside reward them if they poop also reward! thats the main thing make sure you pet them and say good girl or good boy and give them the treat, also use a crate for when you are not home.