As a beginner first learning the ins and outs of keeping an aquarium healthy, things can get a bit overwhelming. There is so much terminology and equipment; it is enough to make anyone's head spin. Learning it all a piece at a time is recommended. Patience is key in many things relating to keeping aquariums. A fish often recommended to beginners is the goldfish.
Commonly available, and quite pretty when healthy, the goldfish is one of the easier fish to keep healthy and happy. Be prepared to do some reading, however. There are terms and procedures to become familiar with.
The aquarium is absolutely critical to being able to keep healthy fish, no matter what kind of fish it is. Proper care and maintenance of your aquarium is necessary, and there are daily and weekly procedures that must be followed. The first step in keeping a goldfish is choosing the aquarium. While you may see goldfish in bowls and other small containers on television and even in real life, this is really a bad idea. While a goldfish can live for a while in a small bowl like that, they will never really thrive, and will be more susceptible to disease and death. A 10 gallon minimum aquarium with filtration should be the minimum you consider for keeping a goldfish.
The filter running into the aquarium agitates the surface of the water and provides more oxygen to the water, which in turn means the fish has plenty of oxygen and is healthier. More room to swim, and even some currents from the filtration, make sure that the goldfish stays happy and in shape. Don't forget to take the number of goldfish into account! More than one goldfish will require more space.
Ask the guys at your local pet store or aquarium shop (recommended) what they recommend.
Don't buy too much tank at first, try to keep it small and simple. Tell them how many fish you intend to keep, and whether you would like to add fish later. Always do some research before buying a product you are not familiar with. There are a ton of accessories and products associated with aquariums and fish. Doing some research beforehand can keep you from buying products you don't need.
Unless you have access to well water, your tap water probably contains chlorine or ammonia.
These substances are very harmful to your goldfish, and can even kill them if you have particularly chlorinated water. There are products available that you simply mix thoroughly into your tap water after filling the aquarium. They usually remove chlorine as well as adding things to the water to help your fish form its slime coat and fight off ick (a small parasite that is extremely common on fish that are stressed). If you would rather use distilled or reverse osmosis purified water, there are other additives that restore needed minerals into these too pure waters.
Make sure you check the filters once a week, and rinse them out with dechlorinated water of some type. It is okay to use distilled water for this. It is common to get snails living on your filters. Try not to crush them, and remove them during your weekly cleaning. It is not recommended that you put these types of snails back into your aquarium, because they tend to over-breed and eat every bit of plant matter in your aquarium.
It is also necessary to keep the aquarium at a fairly constant temperature. An aquarium heater can be used to control the water temperature with some precision. You should also have an aquarium thermometer to verify the water temperature.