Selection of a Tank
Goldfish are very unhappy in a small fishbowl. Small, tabletop bowls are decorative and easy to handle but they were not designed for a goldfish's comfort. The small bowls do not allow the goldfish enough room to grow or swim;do not provide for filtration; and prevent the fish from gathering enough oxygen. A large tank with a hood and fluorescent lighting is the best choice. The fluorescent lights will help any added plants to survivie and the hood prevents the fish from jumping out of the tank.
You should also consider a gravel vacuum to assist with cleaning the tank.
Provide the fish with places to hide by adding plants or other decorative hiding places. Plants are not only functional, but assist with filtration. The plants help beneficial bacteria develop that will purify the water. Since goldfish eat plants, be prepared to replenish your supply. The tank should also contain gravel that has been rinsed prior to placing in the tank. The gravel also helps to trap food and other wastes and must be cleaned when the tank is cleaned. The gravel vacuum can help with this task.
Plants; Undergravel; sponge filters; and canister filters will help to keep the water clean and help maintain oxygen circulation.
Generally, goldfish do not need heated water unless your tank is located in a cold area. Thermometers are also less important since they prefer room temperature water.