We replaced the pressure switch on our shallow well pump. switched on the breaker to the pump and the lights flash and the breaker buzzes,what's the problem here

You have a dead short. Did you wire the switch or is it the plug in type? Did the pump work at all with the old switch? How do you know the switch was bad? What were the symptoms that led you to believe that? The more information you supply the higher the likelihood you will get a good answer.