Ashleybug, I am sorry to hear about the loss of the fish, having a fish for a long time we kinda get attached.
Please know that the water quality is number one and one of the best ways to have good water is knowing how it needs to be.
Some one has to let you know about that because the fish store employee might not understand it, might not care that much, or is simply using information which might be incorrect or incomplete, but thinks they are helping.
Most of those big box stores will take the item back in a return and usually give you store credit or if it hasn't been very long since the purchase, get your money back the way you paid for it.
This is the best basics article which can help you in knowing what to do and how to do it.
The removal of the filter pad will generally mean you left it out to dry or already threw it out. Not left it in aerated treated water.
If the filter pad is all the way dried out it will not be of use to you in regards to having a cycled tank.
You start over with the tank in a number of ways. Usually it would be a complete clean out and sanitize everything including nets, containers for moving water, pitchers and cups, etc.
Quarantining the new fish before adding to your established tank is the better safer option. Sometimes even after keeping fish in QT you could experience an illness in the tank.
You can sanitize the tank and start again. Follow weekly water change maintenance to help keep the tank in good condition, while testing to make sure your water parameters are safe.
You can do fish in or fishless cycle. You can make a new thread and post it here for a link and start anew.