<takes deep breath>
1. the common goldfish can grow to over a foot in length if given adequate housing. This is rarely achieved in normal home aquariums.
The minimum for a small goldfish is 20 u.s gallons and then as the fish grows it needs to get bigger tanks. To house one goldfish successfully in one tank for its life i would say you will need 75 u.s gallons minimum for the first fish and then another 15 gallons per fish after that. Of course people have different views on that and this is just my opinion. These fish do better in ponds and as they are a schooling fish you should ideally keep them in groups.
2. The slower fancy goldfish dont need as much swimming room and one fancy can be kept in 10 u.s gallons for most of its life.
3. filtration is the most important piece of equipment you need. As goldfish are really messy fish you will need a filter or filters capable of at least turning over 10x the tank capacity in an hour.
4. Decor is purely down to choice, gravel is the usual but you must go for a grade that the goldie cant choke on. Pea sized is the best or go the other way and have no gravel at all and have just a few river rocks. Plants and goldfish dont usually mix as goldies eat anything and everything. Plastic plants if you want them. For places to hide, use terracotta pots on their side but try to avoid ornaments with gaps as the fish can easily become wedged.
5. Common goldies can take a wide range of temperatures and the warmer the water the more waste produced and the faster they will grow. Fancies prefer warmer water and especially fish like bubble eyes and celestials need tropical temperatures.
6. Goldfish can also tolerate a wide variance in water chemistry, but they do prefer a high ph and hard water.
7. Goldfish are dustbins when it comes to feeding, they have voracious appetites and are constantly looking for food. Commercial flake is a good staple but it must be supplemented with other foods to keep the goldfish healthy. They like to have vegetables and they love peas. Maybe sometime i will list the types of food to feed them,but they do like variety. Fancy goldfish shouldnt be fed too much dry food as this causes gas build ups and leads to floatiness. Pre soak any flake given and hold under the water to stop fish taking air when eating.
I think thats enough to be going on with, maybe others will add to the list.
hope it helped