Need to Learn How to Stop Dog Barking Fast

My fiance and I live in a small apartment building in the city. We have one small dog, which is fine with our landlady. However, we were just told by a neighbor that our dog barks all day while we are at work. We had no idea. We need to learn how to stop dog barking fast. We do not want our dog to annoy our neighbors. In close quarters like this, it is important to have nice, happy neighbors. We have been looking on the Internet for advice and to see if there are any professionals in our area to help us.