When I started my aquarium a few years back the fish kept dying on me bacause I didn't know what I was doing and how to maintain an aquarium properly. Then after a few talks with my retailer I knew what I was doing wrong and how to fix it. My problem was that I was not testing the water for ammonia levels etc. They told me how the bacteria matures from one thing to another and how each stage effects the fish and what I should do for each stage.
It's a good idea to consult your retailer whenever you have questions. Usually the retailer where you buy your fish from will test your water for free or for a small charge. If they will test it you can just bring a water sample with you to the store in a plastic container and they will test it and tell you what you need to do.
Be sure to keep a record of what kinds of fish you buy so that when you come later to buy other fish you can tell your retailer what you have in your tank and they can tell you what will or will not go good with the fish that are in your tank. It is good to have balance in your tank so the fish will not eat each other, get sick, die, etc. My retailer was extremely helpful to me and they usually are very helpful and will answer your questions.