Barking can be a sign of boredom or lack of exercise. I had the same problem with my dog before and he barks at almost anything. Sometimes he barks when wanting attention from me. How long have you had him?
I know how you feel. I have a min pin/chihuahua mix that is a barker as well. One of the reasons why I will never get a min pin again. They are one of the more yappier breeds, I think. Though she doesn't bark at EVERYTHING. Mostly just people, other dogs, and when she greets me at the door. . lol.