As people gain more experience with goldfish they seem to have fewer disease problems. In general, it seems pretty common to go through a year of serious troubles before one learns enough to stop making mistakes and find a good care regime. And also find durable fish that can live through all this.
Some things that are probably good rules:
Also try to develop some logic for basic diagnosis. Were all fish in the aquarium affected simultaneously? Then check your water quality. Were all fish affected gradually, one after another? Then you probably have an infectious disease. And so on.
A good book to start looking for help in is "The Manual of Fish Health" by Andrews, Exell, and Carrington (Tetra Press ISBN 1-56465-160-6). This has good general info on the ecology and chemistry of aquariums, as well as disease discussions. Also go to my links section and read what Howard Rebel has to say about diseases (for another view).
If you need to approach someone to ask for help, they will need a lot of information about how you are keeping the fish. Diagnosis of fish diseases, particularly without laboratory testing, is very difficult. It is necessary to examine all the parameters of the fish's care to try to come up with suggestions. I have gotten a lot of requests like "my fish is floating sideways. Please help me". As an example, floating can be a symptom of a large number of problems. A lot more info is needed to try to guess what the problem might be.
Some of the care parameters that are needed:
If you send a note to anyone asking for help, you should try to provide as much of the above information as possible in the note.