Answer:
You may want to read up on this, but mostly when you want a dog to stop doing something you just tell them "No." In a firm voice when they do it. Just make sure you don't say it in a kind of baby-talk voice, 'cause then he'll think you're playing around. If he were bigger i'd advise using a training collar as well, but it's not good for smaller dogs. Also, if he stops when you tell him "no", give him a treat. It always works better.
Answer:
Chihuahua's are known as yippers. They bark.. that's what they do. Let him be a yipper. He thinks that he is big and bad, so he is letting people know that.