4 things - good fish, good water, good food, good maintenance. Good fish
: Make sure you select good, healthy fish. Do your homework to select species that will live together well. Pay attention to schooling fish so that you'll have enough of them to be happy. Don't buy fish that grow large for a small tank and don't start out with too many fish at once or crowd the tank later! Good water
: Test kits can be good, but I don't have one. I use a seachem ammonia monitor. Select a good filter and learn about filtration, media and bio-filtration. I believe that good filtration has nothing to do with water flow even though the common myth is you need powerful filters to move a lot of water. Good filtration is slower moving water through fine filtration media.
Tank water is quickly polluted by fish, fish waste, extra food and plant waste. "The solution to polution is dilution" - that means weekly water changes - every week, no excuses. A Python or Aqueon gizmo can make this easy, but for a small tank, a siphon and a bucket may be fine. I use a gravel siphon inserted into a garden hose and out the door to the yard. For the refill, I got a $6 garden hose adapter for the faucet and indexed the hot/cold faucets to give me the approximate correct temperature. Make sure to use a water conditioner to neutralize chlorine. Good Food
: Many spend hundreds on tanks, lights, heaters...then feed lower grade cereal foods. Check the labels and select a quality prepared food and consider treats of frozen foods like brine shrimp, blood worms, etc. Good Maintenance
: The weekly water change is perhaps the biggest best thing you can do to keep fish strong, healthy and growing. Do it religiously. When you do the change, it you don't have live plants, you can use a gravel siphon to also clean the gravel some.
Next is filter maintenance. This you probably won't need to do every week, typically more like once a month. If your water is crystal clear and the return flow from the filter is good, leave it alone. If not, it's time to clean and replace media as needed.
I've only scratched the surface of some of the important things. Like many things, you need to read, learn, experiment and understand. Much of it is just common sense and since you're here, we can help.