The minimum for one goldfish to be happy is 10-20 gallons. If you want more than one, just add another 10. 10 gallons is about 38 liters, so try and get yourself a 40 liter tank for starters, and keep only one goldfish in it. Anything smaller would just cause problems in the long run. This little creature will grow up to 15cm or more if fed well! And don't ever keep it in a bowl. No matter what the pet shop workers say (they're just trying to sell their products), it's not a suitable environment for a goldfish. But if you do insist on having a bowl, buy yourself a betta instead. They have special organs that allow them to breathe air directly from the surface and they're not particularly messy.
Goldfish are very messy on top of that, so you will need to have a good filter, that has a tad bigger capacity. So try getting a filter for a 60 liter tank or so.
You shouldn't need a heater, but buy yourself some kind of thermometer for your aquarium so you can keep track on it (: Goldfish can easily handle temperatures from 19-26 degrees Celsius.
For the prettier things: You could get some nice gravel for the fish. A 2-4cm layer of it should be enough. You could slope it towards the back to give the tank more depth
Suitable plants for goldfish are Java Fern, Hornwort, Elodea and different kinds of Anubias (: (More about plants, filters and aquariums here: (Link is not visible to guests. Please register to view.
Get yourself a few nice pebbles to put around the plant so your fish won't dig them out! Once he grows big, he could start rearranging the whole tank bottom~
Get two different kinds of good quality fish food, flakes or pellets, floating or sinking should be perfect for a staple diet (: Goldfish enjoy digging around for food on the bottom, so if you feed it sinking food that it doesn't notice, you shouldn't worry. It'll be found.
You should feed it about 2-3 times a day and in small doses, so nothing will be left.
If you want your fish to grow nice and big, you should also give it freeze-dried blood worms a few times a week. They're sold in jars in practically any pet shop.
The tank will need to cycle for a couple of weeks before it will be good for your fish to live there, so buy yourself a test kit with pH, nitrite, nitrate and ammonia. Because you need to keep an eye on these parameters while the tank is cycling, otherwise you'll have a sick, or even dead, fish. Ammonia will be piling up the first weeks, so change the water whenever it's too high! When the ammonia drops, you should be testing for nitrites and follow the same procedure. After that the nitrates will be raising slowly, but it can be kept within control as long as you change the water every week (and clean the filter in tank water). Buy a water conditioner to get rid of chlorine in the water and use it for every water change!
For the first fish you might consider getting a hardy variety of goldfish. You can read more about them here: (Link is not visible to guests. Please register to view.
But don't get a common or any other streamline shaped fish, as these need a big tank to thrive! Aim for a common fantail or some other egg shaped fish, these are slower swimmers (:
As creepy as this may sound, it's not too complicated once the tank is up and running (: So don't worry and just get that fish you want!
But do remember that it's a living animal and it needs care and attention.