If your talking about your regular local fish store goldfish. You want a large tank. Those cute little gold fish can grow to 6 - 8 inches and larger. You don't need a monster size tank, but you do want something like 20 - 30 gal.
Don't overcrowd the fish think in terms of 4 to 6 fish in a 20 gal tank.
Goldfish are big dirty fish that throw off a lot of slime. Make frequent large partial water changes.
Goldfish are also a cold water fish and do best in cooler water 65 - 70 F so you don't need a heater.
Keep only goldfish in the tank, don't mix them with your usual tropical fish. Some other cool water fish are ok though.
Feed decent fish foods.
Properly cared for goldfidh can live over 30 years.
Now that will give you a start, but if your keeping some of the more exotic goldfis , You might be better off addressing questions about them in a forum dedicated to goldfish. While we try to help everyone Reef Sanctuary is mostly devoted to SW aquarium systems.
