All well covered by Nossie and Goldiegirl, but I still like to put my "cents" in as well.
First of all - as you mentioned too - what kind of petshop is this where you were sold the fish and the tank without any advice that the tank must cycle first for at least 3 weeks before you can add fish, especialy if you haven't got already some established filtermedia from another cycled tank to speed this cycle up. Those people there are only interested in the almighty dollar, but not in the fishies welfare, hapiness.
In this case I'd go there and ask for the manager and ask for money back.
Second; look for another reputable pet shop.
Third: Very important to get beneficial bacteria and a waterconditioner ( I use "Stability" and "Prime" both from Seachem )
Stability is the beneficial bacteria to avoid the "First Tank Syndrome" and "Prime" removes Chlorine, Chloramine, etc from tap water
You also will need a test kit to test the water chemistry on a regular basis: API MasterTestKit for Freshwater. It contains liquid tests for Ammonia, Nitrite, Nitrate and pH (low and high range)
The Nitrogen cycle was explained before in other posts, I will find the link to it and post it on here too.
When you set the tank up you should have a good filter too, best with 3 or more ( better 5 )times the capacity as the tank volume is. An airstone and a pump to provide fresh air and helps oxygenate the water.
Nice thick layer of gravel where you can plant LIVE plants, not artificial ones as they don't do nothing for the fishes.
Chose a good place for the tank, away from windows, aircondition, direct sunlight.
Goldfishfood: chose a varied diet, different high quality brands like Sera, Hikari, Tetra, OmegaOne, frozen bloodorms. They also eat strawberries, watermelon, zucchini, cucumbers, broccoli, sweet orange pulp and frozen, thawed, shelled peas, all cut in small pieces.
Now when you set up the tank and it is running you can start the cycle with adding fishfood to the water, but no fish if possible. Add Stability daily according to instructions. After 4 days you can add the first fish, just like it is explained on the bottle.
HOW BIG IS YOUR TANK?
Hope this helps, don't be discouraged, it isn't difficult. You and your family will be rewarded many, many times with happy, healty and thriving fishies. They will also show their love to you after a while and giggle,wiggle for you.