The first thing you'll need to look at is the fish tank / aquarium that you will be buying.
Decide what fish you want to keep and how many you want of each species. Remember that not all species are suited to be in the same fish tank.
Once you know what fish (and how many) you want to keep in the fish tank, you can use a simple calculation to work out how big the tank should be to support these fish.
1cm of fish per liter of water.
Please remember that your fish won't necessarily stay the size they are when you bought them, so find out how big each species gets when full grown and use that as your calculation.
Also, some fish, like guppies, may give you fry every 28 days or so, so it is important to make sure that you leave room for these fish in your aquarium also.
Once you have chosen the size of your aqarim tank, make sure that you have enough room for the tank in your home.
If not you will have to buy smaller, and be happy with having less fish, or a smaller number of species.
When buying your first tank always by the biggest aquarium tank you can afford.
The reason for this is:
The bigger the tank the longer it takes for any changes to the water to seriously affect the fish.
It then becomes easier to spot changes in behaviour of the fish and you have more time to react to stop your fish from dying.