Before adding fish to your aquarium you need to cycle the water. This whole process will take about 2 to 4 weeks to complete. Basically you need to get beneficial bacteria thriving in your aquarium to take care of the toxic waste produced by the animals, left over food and decaying plant matter.
PLEASE DON'T USE "HARDY FISH" to CYCLE TANK. It is cruel and will not cycle your tank any quicker. Either buy a cheap aquarium water test kit or find a local pet shop that will test the water for you. Most stores will do this for free. You will be testing for Ammonia, Nitrites and Nitrates.
Fill your tank up with water and run the filtration and air pumps, there is no need to turn the lights on for cycling. Definitely run the filtration because you are going to want the beneficial bacteria to grow inside the media. Keeping the water in the 78-81 degree level with your heater will also help the bacteria to grow quicker. Either put a pinch of fish food in the water or go to the grocery store and buy a shrimp and throw it in the water. If you go with the shrimp method leave the little guy in the water for 3 or 4 days before discarding. Your aquarium is now cycling.
Lets try and to make this short and sweet. The decaying shrimp or fish food will create Ammonia which is extremely toxic to the fish and will cause the fish to die. A bacteria will start to grow called Nitrosomonas Bacteria which will break down the Ammonia into a less toxic substance called Nitrite. High amounts of Nitrites will cause the fish to go to the top of the water gasping and will eventually be fatal. Once Nitrites appear so will Nitobacter Bacteria that eat the Nitrites and turn them into less toxic Nitrates. To get rid of Nitrates you need to do regular water changes or keep a planted aquarium, or a little bit of both.
The directions on the water test kits are pretty straight forward or your local fish store should be able to explain the test results to you. If the store isn't helpful, find another store, they are in a service business and should be able to answer questions and explain things. As the water cycles, first you will see Ammonia levels spike up followed by a decline in those levels followed by a spike in Nitrite levels followed by a subsequent decline in those numbers as the various bacteria grows and cycles your tank. Once your Ammonia and Nitrite levels are undetectable 0 ppm (part per million) or close to it, the tank is cycled and safe for your fish. Nitrate levels are ok at 5-10 ppm but if they get in the 25-50 ppm area you need to do a water change.
Hope that this was helpful and feel free to contact me and I will be happy to answer any questions you might have. Enjoy your new aquarium.