Be as specific as possible.  By providing the 9-1-1 call taker with as much accurate information as possible you will ensure that an appropriate response will be dispatched to assist you.  Remember that 9-1-1 is for true emergencies.  You should always call 9-1-1 if life or property is  in immediate danger.  If you aren't sure that the situation is an emergency, err on the side of safety and dial 9-1-1.  Let the expert who takes your call determine whether you need to be directed to a non-emergency number.