Hi and welcome to the forum
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You don't need fish to cycle a tank and in fact cycling a tank with fish has a lot of drawbacks in comparison to fishless cycling. 6 goldfish will produce a lot of waste and is too many to add to a new tank of that size at once- your main problem now is controlling the ammonia levels which will be quite high in the tank right now.
You should definitely invest in water quality test kits for ammonia, nitrites & nitrates as the bacteria that cycles the tank is a type of beneficial nitrifying bacteria and the cycle it goes through is known as the Nitrogen Cycle. It needs a constant supply of ammonia to survive (provided by the fish), undechlorinated water (you can neutralise chlorine & heavy metals found in tap water with dechlorinator), oxygen & a current to survive (provided by the filter, on which its media the bacteria lives on).
The beneficial bacteria consumes ammonia (which is very toxic to fish) converting it into less harmful substances. The process goes Ammonia - Nitrites - Nitrates. When the tank is fully cycled the bacteria will work in harmony with the ammonia load of the fish so there is no ammonia present in the tank at all. But the bacteria takes a long time to establish itself and this process is called cycling, and until your tank has finished cycling it will experience very unstable water quality with first ammonia problems (very toxic to fish) then nitrite problems (less toxic to fish but still toxic).
When the tank is fully cycled (with fish you're looking at about 2months+) it will have 0 ammonia 0 nitrites and some reading of nitrates (the end product of the nitrogen cycle, and not toxic at all to fish unless in very high levels- ideal levels would be between 1-40).
Even the slightest reading of ammonia (or nitrites) is bad to fish so you must keep the ammonia & nitrite levels under control until it finishes cycling. Stress from water quality problems is one of the biggest causes of fish deaths and root causes of fish diseases in aquariums. The best way to safely lower ammonia or nitrite levels is to do regular partial water changes (taking out between 15-60% depending on the severity of the problem) with dechlorinator. The more fish you have (and the higher waste producing the type of fish you have plus their overal size) when starting off a completely new uncycled aquarium, the worse the water quality problems will be.
I would assume at 3ft long your tank is about 30 gallons but it is important to figure out the exact amount of gallons as knowing the volume of water that your tank holds and its exact measurements is crucial to knowing what & how many fish are suitable for your tank. Goldfish are hardy fish but 6 is too many for a 3ft tank and they are notorious for being high waste producers (they can eat up to a third in their body weight in food a day) and so while they might survive the impending water quality problems ahead, with so many you might have to partially water change the tank every other day just to keep the ammonia/nitrite levels under control.
Here is a link to the forums index on everything you need to know about cycling tanks and the best ways to do it (including fishless cycling, where a pure source of ammonia is used instead of fish to cycle the tank, which has many benefits and is much quicker to do);
http
/www.fishforums.net/index.php?/topic/277264-beginners-resource-center/
You will find that having less fish in the tank to cycle it or even taking the fish back and going for a fishless cycling route is much better for both you and the fish. Pet shop advice is so-so, so buyer beware
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to the forums! I'm glad that you found this site because it is one of the best you'll ever find. If you need help go to the beginners section of the forums. Also make sure to check out this link. It will show you how to cycle your tank the best and fastest way possible, fishless. I hope that you feel welcomed. Also don't forget to ask alot of questions, after all that's what we're here for. 
