The right way to add a fish in the tank is that you must afloat the fish in a bag in to the aquarium for around 15 minutes prior to leaving them in the tank. This would help your fish get acclimated to the temperature of the water in its new home.
Make sure that you do not add the water of the fish bag in to the aquarium as this water might be contaminated, or would at times affect the temperature of the aquarium and the P.H. level of the water in there.
Before giving the first ever feed to the fish, give it at least two hours to get acquainted with its new house.
While purchasing the first fish of yours, be sure to purchase some food for it. Remember that you should feed only small amounts of the food initially.
The food that remains settles down at the bottom and rots down finally. This rotten food becomes debris and develops severely toxic chemicals that can kill your fish that is ammonia. So, try to maintain the least amount of remaining food. This thing must be taken are of in the newly set up aquariums especially those that lack the nitrifying bacteria.
The trick to do it simply is to watch your fish while feeding them. Only put in the amount of food it consumes in the first 2-3 minutes, that to only twice a day. In case of food, also make sure to read through the label of the fish that you always get on the purchase of the fish.