You need to go to a place that sell fih and get a treatment to put in the water. It may be too late for the fish that is infected but should stop the diease from spreading to the others. If the fish has been injured they often get a fungus growng on the wound but it needs to be treated quickly.
So long as it's a common bacterial infection then the fungus can be treated easily. Describe the symptoms to your local pet shop and they should be able to provide the correct treatment. You will need to treat the entire tank so follow the directions carefully. If your goldfish doesn't improve then you will have to put it into a quarantine tank before trying any further treatments.