Your dad has a 40+ gallon tank, right? Does he not have test kits to check the water parameters? That would be one of the first things to invest on. Before you can put a single living thing in your tank, you need to know what the water parameteres are and if they are even suitable from living things.
Take a deep breath... take the fish back to the LFS, do a large water change on the tank. You haven't lost the war yet, just taken a few steps back. You can get everything under control, just slow down and read up on the nitrogen cycle. You can do it!