Goldfish is being eaten alive help?

If your goldfish is not being attacked by a tank mate (which would mean that you either need to remove the aggressor, or place the beaten up fish in another tank) then it's likely a water quality issue.
Typically when something like ammonia is at toxic levels, this will begin to shred & rot your fish's fins.

If this is the case, you need to do an immediate 75% water change in your tank making sure that the new water is the same temperature as the old water, and that you use the recommended dosage of aquarium water conditioner per the instructions on the bottle.

You should make sure that you are doing regular maintenance in your tank which includes at least a 50% water change every week; more if your tank is too small or overstocked.