To set up a goldfish tank, let me define tank. Most people, when they buy a goldfish, when they win a goldfish at a carnival, it usually comes in a bowl. I think goldfish bowls are one of the top sellers at aquarium shops, so let's talk a little bit about bowls, and then let's talk about why it's better to get a tank versus a bowl.
The bowls, to set them up, you just want to put one fish in a bowl. The bowl should be at least a gallon in size, preferably 2 gallons. Put a little bit of gravel in the bottom of the bowl so you add a lot of surface area for bacteria to grow. I also recommend buying a little bit of bacteria, a net, some food, some dechlorinators, and a pH test kit. Just because it's a goldfish in a bowl doesn't mean the rules that apply to aquarium keeping don't apply to a goldfish. It's a living thing, and it has the same needs as tropical fish, just a slightly cooler temperature.
So you set up the goldfish bowl. Use water that's been filtered, or Poland Spring or Deer Park water, and let it get to room temperature. The reason for using filtered water is that you don't have the gas that's associated with tap water. Remember, tap water is under pressure, and when the water is put in a bowl, you've got all the bubbles that build on the sides of the tank. Well, those gas bubbles are lethal for fish, so you want to let the water sit overnight if you're going to use tap water. You also don't want to shock the fish with water that's too cold or too warm, so it's best to take that water and let it sit and let it become room temperature for four to five hours, overnight is even better, before putting the fish in the bowl.
The reason why you want to put one fish in the bowl is because oxygen is going to be very limited. Even though goldfish can scavenge oxygen or air from the surface, you don't want to overcrowd them. You want them to be able to get some of their air from the dissolved aqueous environment, as opposed to having to scavenge all the air from the surface. You also don't want to overcrowd it because you don't have a good filtration system on a bowl, so the bacteria that's growing in the gravel can only process so much waste.
Do your water changes once a week. Don't overfeed. Feed the fish once a day, as much as it can eat in five minutes. What that means is you just put a little pinch of food in there. If it eats all of it, put another little pinch. If it eats all of it, keep doing that for five minutes. The goal is to not have one leftover flake at the bottom of that tank. If you have leftover food, you should take a net and scoop it out, because in a bowl it could result in cloudy water or water with a high ammonia level, which will actually burn the fish's skin; pretty nasty.
For all these reasons, I recommend going with a tank. Try to be strong and don't go with that bowl. Get a five or a ten gallon tank. Get a basic hang-on filter, or an under gravel, or an internal sponge filter, anything instead of a bowl. You do want some type of moving filtration. It'll make the tank run so much better, and you can put a few goldfish in it. But just remember, the goldfish get big, so even a 10-gallon tank is going to be too small when that fish is four, five, six inches big.
But to set it up, you don't typically need a heater for them. They do prefer colder water, so room temperature water is fine, unless you live in a very cold area, and it's in a cold room, and you go away, and there's no heat in the house. Then you'd want a heater in the aquarium, because you don't want the water to go from the 60s down to the 40s. That will just stress the fish.
So you need basic decorations, gravel in the tank. You don't need an aquarium light. You don't typically need a cover on the tank. Goldfish usually don't jump, but sometimes they do jump, so be prepared to find your fish on the floor. I know it sounds morbid, but that is a risk that you have with an open top aquarium. Also, if you have cats, as you know, cats love fish. They're attracted to them. So if you have a cat, you'd probably want to go with a strong, tight-fitting top to the aquarium.
Those are the basic tips that you need for setting up a goldfish tank.