Sadly you got some bad advice from the fish store.
I will assume now that you do not have any livestock in the tank which is good, any other livestock will stand a good chance of dying as well. since you have ammonia in the tank do nothing and let the tank cycle. eventually the ammonia will turn to nitrite and then nitrite to nitrate. Once the ammonia and nitrite are down to zero, this may take a few weeks, then you can do a water change and slowly begin to add livestock.
A few suggestion:
Purchase some test kits for the ammonia, nitrite and nitrate. BTW what is your ammonia level now??
while the tank is cycling read up on quarantine procedures and Tank transfer method. to prevent the introduction of parasites and diseases.
At the top o the forum page there is a sticky called setting up, look for the big red arrow, it is full of useful information.
From now on I would be skeptical of any advice given by that particular fish store.
In the future when posting try to give actual testing numbers and pertinent tank equipment info. this will help give more detailed responses.
Good luck just my 2 cents.
I forgot, welcome to the forum.