Say you want to execute a method called shoot() when the player presses the left mouse button on the screen. To do this, you would need to look at the EventType object in an Event in the queue and determine if it's type is MOSUEBUTTONDOWN. Then, if it is, check the button attribute on it and make sure it corresponds to the left mouse button (which is 1).

for event in pygame.event.get() #this block iterates through the queue
	if event.EventType.type == pygame.MOUSEBUTTONDOWN: 
		if event.EventType.button == 1 #if the button pressed is left mouse button
			shoot()