Before starting with your aquarium there are 3 factors that you should consider:
1. Your tank dimension
The tank size will depend largely upon the type of fish that you are going to have. For a beginner i would consider that you should buy goldfishes as they are more resistant. A goldfish will survive long in cool-water aquarium. They should not be kept in heated water. For the tank size i would advise you to buy one of an average size, i.e of about 36"*18"*18" but of course a smaller one would do. You should also know that the bigger the tank the bigger your fish will grow as fishes grow according to their environment.
2. Aquarium equipments
To remain healthy and live long periods your fishes will need equipments such as filter, lights, heater, ... to give them a
feel of living in the natural temperament.filter
You will need to have a filter equipped with an air pump and also a non-submersible air pump(which you place outside the tank). During day time you will use the submersible pump and at night you will use the non-submersible pump. This will help in terms of your pump lifetime and also your fishes will be apt to better rest as the pump there will be less current in the tank.
Heater
You will have to choose the brand of the heater according to your budget. A large variety of heaters will be available at your local pet shop. However you should not forget to include a thermometer to measure the heat fluctuation.
Lights
Lights in an aquarium are very essential, not only because it adds beauty to the tank by making it more attractive but also influence your fish. The lighting of your tank will have an effect on the colour of your fish.
There are some types of lights available. The most common are strip lights, full hood lights and canopy lights. There are also incadescent and fluorescent fixtures. I would personally recommend fluorescent fixtures as they produce more light and less heat than incadescent.
3. Accessories for your aquarium
The accessories in your tank also plays a big role for your fish. Suitable accessories like plants, gravels, rocks makes them feel that they are living in a natural environment. For instance each fish has a different character and attitude. Some are shy and so would prefer hiding behind the plants.
You have to be careful while choosing the ornaments for your aquarium as some may wound the fish and cause diseases. If you decide to buy bubble-eye goldfish or the lionhead you will have to be more cautious as they are more vulnerable. You should buy only items labeled as "Aquarium safe" to prevent any bad "surprises" as some may be toxic for your fish. Safe one should be available at your pet shop.
Gravels, rocks, plastic plants.. and also collect rocks from your garden. But make sure that you clean everything thoroughly with conditioned water before putting them in your aquarium so that to remove any pesticides, chemicals or any harmful minerals.
Note: If you are having goldfishes use plastic plants instead of natural ones as goldfishes are great plant eaters.
Note: Avoid putting materials like corals, saeshells in your aquarium as this will produce carbonate which will decrease the water PH.
To check if your gravel is inert, just put some vinegar on it and if it foams or bubbles then the gravel is not inert.
Adding water & fishes to tank
Then fill your tank with water, leaving your pump switched on, and wait for about 1 week. This will ensure that there is no chlorine and that some bacteria have started to develop. These bacteria will be useful as to digest the fish wastes. Finally, you can put your fishes in the tank. If you decide to add tropical fishes like comets, koi.. then include only this type of fish. Also you must distinguish among the fish pace. Some are rapid and others less rapid; for example if you put a lionhead goldfish and a comet goldfish together, then the lionhead may find difficulties in finding its food.
That's it! You can enjoy your aquarium.
