How to train a dog not to bark at everything?

While reward based training works, it doesn't work without consequences. What you need to do is to train this dog to watch you. You need to make that his job. Any time he starts to pay attention elsewhere, turn around quickly and walk the other way. You must do this before he starts barking. Do it during the alert phase. When he catches you praise and treat. If you must drag him you are correcting him too late. The turn must be performed before the dog becomes amped up. A good trainer or training class will help you a lot here. You need to start this exercise in a quiet place at first to get the dog to watch you and to get your timing down to a fine art. as he becomes more attentive to you, then you can start adding distractions.