The lady at the pet shop was out to make a sale...and she did. You are not the first person to be mislead by the supposedly helpful person at the pet store. She has set you up for disaster...now you have to step up and get educated, fast, or your fish will not survive.
Read up on the nitrogen cycle of a fish tank. Get a test kit (drop type) NOT test strips (they are very inaccurate), and test your water for pH, nitrite, nitrate, and ammonia. Post the numbers here, and we will help you thru the cycle. Get ready for lots of water changes to keep the ammonia in your tank low enough to (hopefully) not damage or kill your fish. Taking out 15% every few days is NOT going to do it. Most likely you will need to do 50% or more EVERY day, until the cycle is done. Which will take several weeks. Fewer fish, or a bigger tank, (or both) would be easier on you and the fish, but if you want to save them you must get busy and get educated. Do you know about chlorine removers? Get some Prime (it helps neutralize some of the ammonia, and is less expensive than many others).
Do a large water change now (at least 50%) with a chlorine remover, and be sure to match the temperature of the new water to the temperature in the tank, everytime. Get the test kits and post your numbers here. Get educated QUICK about the nitrogen cycle. And consider giving the lady at the pet store a serious piece of your mind...she has done you NO favors!