Before you rush out to buy the goldfish tank, you need to consider where to put it and what to put it on - Every litre of water weighs a kilogram so a metre long tank can weigh as much as a big man - for safety, rather buy or make a special stand able to take the weight of the water..

Firstly, it needs to be away from continuous sunlight.

This is for 2 reasons:
(a) Lots of sun warms up the water, and your goldfish is happier in cool water, and
(b) Algae grows really fast in full sun - you don't want to spend time each day removing algae so you can see your pet !

Secondly, keep the tank away from doors.

Another couple of reasons:
(a) If the door slams, your fish is going to get one heck of a fright !
(b) Humans are substantially bigger than goldfish - we are giants by comparison - and a giant apparently leaping up in front of a goldfish, appearing from nowhere as far as the fish is concerned, will have him shaking in his fishy shoes. Let your fish see you coming into the room.
Remember that a nervous goldfish is likely to become a sick goldfish.

Thirdly and finally, it will be convenient to have an electrical plug or two nearby as you will need to at least have a water filter of some sort running, and that needs power. You might also want to have a light for the tank so you can see your pet in the evenings.