OH, crud. Well, do you want advice on how to sterilize the tank so that you will be ready to go when you decide to try again? Also, lots of people here can help you get thru a fish-less nitrogen cycle, so that your next fish will have a much better chance. I can't help you much with that, having had the same tank set up for 8 years now!
This forum is a marvelous place for information. If I may throw in one more suggestion...do not believe everything that fish store personnel say. Get your info here, where no one is trying to sell you anything.
I hope your next aquarium adventure goes much better!