The first important step before putting any fish in an aquarium should be to learn about the nitrogen cycle. The nitrogen cycle is the process in which toxic waste materials in your aquarium are broken down into a less toxic state.
Simply put, Fish waste, uneaten food, and decomposing plant and animal matter break down in the aquarium as ammonia. Ammonia is broken down by "good bacteria 1" as nitrite. Nitrite is broken down into nitrate by "good bacteria 2". Nitrate can either be used as food for any live plants in your aquarium, or simply removed/diluted by doing regular water changes.
Without the nitrogen cycle in an aquarium, fish could not survive very long. The buildup of ammonia, nitrite and nitrates would reach toxic levels and poison the fish.