These are just my opinions on keeping fish, possibly someone can disagree or correct my line of thinking, as I am not an expert.
Fish are swimming around in their own waste, which is probably the reason most fish die. So what can you do:
1) Don't over feed, reduce the amount of food you give them. I took my last tank down to 1 feed per week and the fish lived for over two years, until I bought some new fish, then all but the upside down cat fish died.
2) Get a Big biological filter, as your local water authority may use for your waste.
3) Change water regularly, but with tap water you add chlorine (and possibly other chemicals), this is not good for the fish. I have a pet hate for using chemicals to get rid of other chemicals, so I air-rate the tap water as much as possibly to let the chlorine evaporate.
4) Use better food. Fresh live food is the best, but frozen food is great. You will also notice an improvement in colouring in your fish.
Another problem I found was introducing diseases with new fish. You have a good setup, plenty of well established fish, so you decide to buy that nice sword tail in the pet shop. Next thing you know half your fish mysteriously die. So what can you do.
1) Find a shop with not only a fish mad aquarist, but one that displays active disease control. eg, quarantine tanks for new stock.
2) Have a quarantine tank of your own.
3) Don't fix it if its not broken, in other words, if your tank is settled don't interfere by adding more fish.
Rogue fish. There is always the odd fish that will just keep killing its fellow tank mates. You can't train fish, so what can you do.
1) Get it its own tank
2) Rehome to lfs
It is a good idea to pick fish for your lifestyle. If you are very meticulous and don't go on holiday, then you could probably look after a marine setup. If you are a child or worse still a teenager, then stick with a tough as old boots gold fish.
It's also good to read books and ask experts. Knowing the temperature range, ideal pH, whether the fish is fresh water or brackish and what environment it comes from (river, lake, etc) helps.
*I edited this as some of this is wrong enough for the hobby.