Do take some time to read thru the postings on this site, especially about "cycling" the tank.
This process is something I never knew about and was a real eye opener for me.
I too took the bad advise of a Petco guy then I found this site.
I actually thought that fish dying after a few days,weeks or months was a normal process, wrong!
They should live for many years with proper care.
I wouldnt be at all surprised if your dead fish might have been caused by deadly toxins in the water.
If you want to take the best measures to keep your fish healthy then you need to invest in a much larger tank (30 gal+), a good filter system, a test kit and a host of other things.
Everybody here is very willing to help with any information you need.
Oh, and be careful with advise from friends and family who have had "fish for years and never had any problems".
Fish keeping is an enjoyable hobby.
At first the setup may seem alot more trouble than its worth but once you get a routine figured out then you can watch the fish flourish.