It depends upon the nature of the break- if it is just the frondy, fin section of the tail that has been damaged then you don't need to do anything as it will grow back naturally. However, if the break is higher up the fish's body and extends to the tail end of it's backbone, then it's best to consult your local vet or aquatics stockist (or, if you have access to one, a marine biologist). They may conclude that the fish can live quite happily with the break and advise that it just be left, or that it would be kinder to put the fish to sleep, which is most humanely done by putting it in a plastic box of full of water and placing this in your freezer, which will send the fish to sleep painlessly. It is very difficult to operate on a marine animal as small as a goldfish, as they cannot be anaesthetised.