When you go into the pet store look at the fish tanks; think about the answers to these questions. Do the fish tanks have dirty water and is there stuff floating in the water? Look at the gravel on the bottom of the fish tank. Is there old rotting (decaying) food in the gravel? Do you see any dead fish floating? If this is the case, go to another pet store. You want to start with healthy fish.
When you pick out your fish look at the tail and fins for signs of splitting or patches of scales missing. Look to see if you see any injuries. Is the fish swimming with its head downward? These are all signs the fish may be sick or injured.
Larger goldfish are usually hardier than smaller goldfish. But, remember you can only have so many fish in one tank otherwise they will be over crowed and not have enough oxygen to breathe. Go back and read the first goldfish article so that you can remember how many fish can go in what sized tank. Also, you don’t want to buy more than three fish the first time. So You Want a Pet Goldfish
Also, only get goldfish, not other fish; some fish could hurt your goldfish. A fish other than a goldfish may not be able to live in the same environment.
Fast temperature changes kill millions of goldfish. Fish are cold blooded (like snakes and lizards). They have no way to control their internal temperature. This is the reason it is important to keep your fish tank at the right temperature for the type of fish.
When you receive your fish they will be put in a plastic bag with some water. The bag will be about 1/3 full to make sure there is enough oxygen for the fish to breathe. When you buy your fish make sure you are going straight home. If you wait too long the fish will suffocate. The fish will need to spend time in the bag at home too. The total time involved will be the trip home, plus a half hour at home. The bag with the fish will be placed inside the tank; the fish will have to adjust to the temperature of the tank before it can swim out. 
After the half hour is over you can let the fish swim out of the plastic bag. Have someone help you hold the bag in the tank and then just cut the top off of the bag very carefully. Allow the fish to swim out of the bag; don’t rush it by dumping the bag.
Now you will want to feed your fish, but remember not to overfeed the fish otherwise the leftover food will rot, bacteria will form, and the fish will not have enough oxygen to breathe. Remember the article that explains feeding your fish. Setup Your Goldfish Tank It is a good idea to read this article again. 
Once you make the decision to have a goldfish or any pet, you have taken on the responsibility to care for this pet. Above all else, enjoy your pet goldfish!
