First of all, PetCo employees rarely know much about keeping fish.
Secondly, good filtration is a must. There are lots of different ways of going about this, but the easiest and most affordable way is to use a hang on back filter, like the one you apparently have. Tanks require time to cycle, so frequent partial waterchanges are a must for the first few weeks. Change 25% of the water daily for the first week, and then every other day for a week, then every third day, etc, until you are changing 1/4 of the water every week. After that your filtration bacteria should be well established and your filter media will be doing it's thing. For fish only tanks, bright lighting is not necessary, and neither is a 12 hour photoperiod. Keep the thing out of the sun.
Don't be tempted to buy a new filter insert every month. Used filter pads are home to most of your beneficial bacteria. Keep yours in there, and if it needs replaced, retain a good portion of your old one to "seed" your new filter. Backwashing in aquarium temp water is the best way to keep an old pad working.
Make sure you change 10-20% of the water at least every other week, and don't put too many fish in there. Don't put incompatible fish together, and avoid delicate or needy fish. DON'T buy a fish that gets to be 24" long, but looks really cute in the store, such as Tinfoil Barbs, Ocsars, Pacu, Knife fish, etc...
A simple Siamese Fighting fish setup should be very easy to maintain.
How big is the tank? What are all the occupants?