Bulging eyes on a goldfish are a symptom of an illness called Pop Eye which is basically a bacterial disease.
Before treating the fish I suggest you have the water tested for levels of ammonia, nitrites and nitrates. Ammonia and nitrites are harmful to fish in any amount and will cause a fish's immune system to be lowered thus opening it up to a variety of diseases.
Nitrates are harmful only in high amounts. All 3 are caused by fish waste and decaying matter in the aquarium and ammonia/nitrites should not be present in an established fish tank.
Test results should indicate ammonia and nitrites=-0- and nitrates 20ppm or lower.
If you don't have a test kit you can bring a water sample to your local pet store and they will test "au gratis". If the test results indicate a problem then a series of daily 15% partial water changes will be in order accompanied by a drastic cut down on feedings. Once test results show the levels recommended above you should then begin medicating the fish.
As for a treatment plan. Begin by changing 25% of the water. Once done add a product to the water called Stress Coat, click on link below for a view of the product, as it will help heal the affected area.
After the partial water change has been done and the Stress Coat added begin to medicate the fish by adding a medicine to the water called Maracyn, click on link below to view product, which can be easily found at most major pet stores. Follow the directions on the product label for usage, dosage and length of treatment.
It would also be helpful to isolate the fish in a smaller hospital tank prior and during treatment.
Good luck and please don't hesitate to ask if you have any questions.