Goldfish

Housing
Before you go out and buy your goldfish, buy a 10-20 gallon freshwater tank (yes, you heard me, they like to swim around in a large space and poop a lot. If given proper space they can live for decades.) You can go a little bigger or smaller, but whatever you do, do NOT get a fishbowl. It stunts their growth, and they could die prematurely, goldfish on average can live 10-30 years. As well as the tank, buy gravel big enough so that your fish cannot eat it, a filter appropriate for your tank size, a hood that fits your tank, and a light (Obviously.) Also, after you get all your equipment, and if you are using a tank used before, do water  changes to see if any dangerous chemicals, such as ammonia are there. Do this for at least a month before buying the fish, or until the ammonia levels reach 0. Ammonia is a chemical that can be dangerous to the fish.

Feeding
Goldfish should be fed fish food specifically for goldfish (goldfish food) and should only be fed once per day (you can feed them once every other day if you really want to.) If you overfeed them, the goldfish food will collect at the bottom, and decay into ammonia.

Cleaning
Even after the ammonia levels (see "before you buy") are at 0, you still want to do water changes once per week no matter what. You never know.

Other information
-Before you get another goldfish, get a bigger tank (unless your tank is big enough for two goldfish, i.e. 30 gallons or bigger) and make sure the new fish is the same size as the other goldfish (if it is smaller it may get eaten, and if it is bigger it may eat the other fish!)

-Goldfish can be taught tricks! You can buy a fish training kit, the best known one is the R2 Fish School Kit, it can teach your goldfish to "kick" a soccer ball into a goal, do the slalom, or "kick" a field goal!

-There are many different goldfish breeds, like fancy and common. Fancy goldfish generally need more space than common, due to their bigger size. They're also more expensive

-Make sure you put an anti-stress coating in the tank water, to ensure the safety and longevity of your goldfish. Read the instructions on how much you should put in, too much could kill the goldfish.

-The Guinness World Record for longest living fish is a fish named Tish, that lived for 43 years.

Image credits
http://www.aquariumslife.com/freshwater-fish/cyprinidae/common-goldfish-carassius-auratus/

http://www.tropicalifish.com/red-cap-fantail-fancy-goldfish/