Diana,
If the goldfish you have is indeed a Common (or "Comet") goldfish, then he will require lots of room. I do have Goldfish and mine are mostly fancy goldfish just because they are more likable. They dont steal the food from everyone else, they dont fight as much as the commons, and they just look better! IMO
Anyway, here is a little information on common goldfish:
Tank Requirements:
Common Goldfish require 20-30 gallons minimum. They aren't like fancy goldfish, which can be perfectly happy in 20 gallons or so. I have heard of people who say that they had to upgrade their 30 gallon tank because the ONE common goldfish they had in their tank was outgrowing it!
So, if you have the money you should ideally have a 30 gallon tank. I only say this because if you start with a 20 gallon and spend all the money on it now, you might be looking to upgrade it in just a few months. These fish grow quickly, and aren't very forgiving as far as allowing us to cramp them in small spaces.
Heaters:
Common Goldfish dont "need" a heater like tropical fish do. A steady 70 degrees F. will due, but the key idea is to keep the water temperature "steady". If you dont have a heater in your tank, your tank could end up fluctuating too greatly, even for Goldfish. I would test your tank before adding the goldfish, check the temp at night and then in the mid-morning to see how much it changes. This will serve as a guide.
Tank Cleanings:
Goldfish, as you know are extreamly messy fish. They produce lots of waste, and can really do damage to an tank which isn't regularly cleaned. Ideally, you should be doing 40% water changes every 7-10 days. You shouldn't go longer than 14 days without doing a partial water change of some kind. Make sure that you use your gravel vacuum when doing a water change. You dont need to do a gravel vac. every time you do a water change, but you should be doing this at least 2 times a month.
Plants:
I have always kept some plants in my tanks. Some of my goldfish LOVE to eat and munch on them, others do not. It's odd really. Test out some inexpensive plants and see what your fish does to them. If he/she leaves them alone for the most part, and doesn't seem interested in uprooting them, then you may add some plants to his tank. It's really a personal preference, and plants aren't 'needed', although they do have their own benefits.
Life Span:
Common Goldfish, if cared for properly with a nice home (clean water), and decent food can live well into the teens. Many advanced Goldfish keepers have reported their goldfish living 20+ years. The bottom line is, if your goldfish dies after 3 years...or even 5-6 years for that matter, something went wrong. This should not be wrote off as "old age".
--------------------------------------------------------------------
Well...I think I covered just about everything you need to know about the basics of common goldfish. Advanced reading can be found at a number of places. Here are a few of my favorite links for these wonderful fish:
My Favorite Links:
Analysis of Diseases/Troubles (Primarily Coldwater Fish)
Why Goldfish NEED All That Room!
All About Algae
Fishless Cycling Info
Good Luck!
Will tropical flake be okay for a day or two? I hear they are major poopers- i need excessive filtration? 
