If your dog has something in his mouth from the garbage, and you confront him with a face-to-face approach or a hand to grab the object only to be met with an aggressive response, there’s likely some resource guarding involved. Your dog needs to be taught that good things happen when he is approached or when a person takes something away from him. This is done step by step, little by little and in a non confrontational way. You need help from a humane, positive trainer who knows how to deal with resource guarding. You can read up on resource guarding and try to help your dog by yourself at first. The book “Mine!” by Jean Donaldson is good too.