How do you get your dog to bark when the doorbell rings?

How do you get yoru dog to bark when the doorbell rings. We have a 6 month old golden lab and we want him to go to the door and bark when the door bell rings. He hardly ever barks to begin with. He barked once and it scared him. I know-- wierd. Anywho. The only tricks he has learned is to sit. How do you teach him to bark at the ring of the doorbell. Please give me suggestions. Thanks.