If you want a happy goldfish get a 20 gallon fish tank with a filter recommended for a tank 10 gallons larger (if you get a 20 gallon fish tank get a 30 gallon tank filter). Goldfish are very messy fish so make sure you do 25 percent water changes every 2 weeks. DO NOT use one gallon fish bowls. Those are cruel, no fish should EVER have to live in one. Not even a betta fish.
Here's a list of other things you might want.
-Filter
-Replacement filter cartridges
-Gravel
-Live/plastic plants *
-Gravel vacuum**
-Goldfish Flakes
-Aquarium bucket( for water changes) *
-Water test kit **
-Water Conditioner
-Decorations*
*Optional
**Optional, but highly recommended
With all these conditions met, your goldfish could live 10 years!
If you don't really want to poor a lot of money into it then I suggest you get an Eclipse System 6. It's a 6 gallon tank that needs:
-Replacement filter cartridges
-Gravel
-Live/plastic plants *
-Gravel vacuum**
-Goldfish Flakes
-Aquarium bucket( for water changes) *
-Water test kit **
-Water Conditioner
-Decorations*
*Optional
**Optional, but highly recommended
Good Luck!!