Welcome to the forum! I am sorry that you have had so much trouble, especially from a fish shop that doesn't understand much about the fish they sell. I would advise three deep breaths and some reading (ok, a lot of reading on this site). What I (we) want is a life-long discus hobbyist! You will get much help and suggestions here. Everyone of us have made mistakes along the way.
However, first of all, check the Discus Beginners section and the sticky topics. They have one for setting up your first tank, along with many other helpful topics.
Here is another must watch from Al, the owner of this site.