Hi, I'm sorry for your loss. You probably experienced an ammonia spike. New tanks have to do something called "cycle". Animals (including fish) release ammonia as part of their waste products. Food and other organic materials also break down and release ammonia. When a tank is cycled the ammonia gets converted by nitrites by certain "good". Both ammonia and nitrites are harmful to fish, especially if the levels are high or if they are exposed at low levels for an extended period. The second part of the cycle is the growing of a second type of "good bacteria" that convert the nitrites to nitrates. Nitrates are also harmful to fish, but not as "deadly" - nitrates are the "end stage" of the "nitrogen cycle" and it is to remove / dilute the nitrates that we do water changes.
I know , it all sounds like mumbo-jumbo right now but don't worry - the folks on this forum will help you through this.
Here is an article on cycling that explains the whole process in more detail (the same process applies to both freshwater and saltwater, so ignore the saltwater references)
Since you already have fish in the tank you have two options right now:
1. return the fish and do a fishless cycle or
2. do a cycle WITH fish - it takes longer and its more work, but if you can't return the fish its probably your only option.
You should buy the Aquarium Pharmaceuticals Freshwater Master Test Kit. You can find it at petsmart (if you go online and print out the page for the product and bring it to the store you'll get the online price, which is lower). This will allow you to test for ammonia and nitrites as well as nitrates. Whenever the levels rise you'll need to change out some water (as much as 50%) to dilute the levels.
In the end its worth it - we've all made "new tank" mistakes .... just hang in there and we'll get you through it