This article and the links included in it, might be a good resource for you.
Guide to Starting a Freshwater Aquarium - Aquarium Advice
One of the biggest things which helped me was understanding the cycling process, and how BB / beneficial bacteria live in the filter pads/media, and you basically NEVER throw out filter pads or rinse in plain tap water (use pwc
water or treated/dechlorinated water).
This info was life changing for me AND my fish!!!
When the tank is not cycled (meaning it has a mature colony of BB), it causes fluctuations and toxic conditions in the tank which kill fish.