The first thing to consider when you are planning a new aquarium is where are you going to place it? You will obviously need to be able to see it, whilst having easy access on the water changes to remove and replace the water. But, whilst all of this water is being moved round, you also need access to electricity (with a lot of plugs!). Given that water and electricity do not mix, a few thought is needed.
Then, with a lot of fish, if the tank is in a high visitors area of your household, plenty of varieties will not take kindly to the constant disturbances. You will also want to stay away from draughts and direct sunlight. So in all, quite a conundrum when you are trying to work out where to position the tank.
Now, you need to start on the supplies! Along with the actual tank you need the cupboard to place it on. A large tank will weigh stacks - even a two foot tank might weigh more than a grown man. So the furniture that you place it on should be the proper equipment, not merely any old bit of a cupboard that may not take the extreme weight.
Next, look at the equipment that is needed to fit out the tank. You might find that your aquarium arrives with much of the gear that you will need, but stacks of the time it is not actually up to the job. Let's say, you need a decent filter to keep the water clean. But in my experience, the filter that arrives with the aquarium is just only up to the job if you have no living plants and merely several clean living fish. Start building up to a full aquarium and loads of the required plants and the load on the filter is far too much. I always upgrade a filter to at least the next size up from that which is recommended for / supplied with the aquarium.
If you have bought a general aquarium then it may be setup for cold water fish. The conversion to tropical fish is quite simple, as long as the aquarium is made of suitable materials (you should not try to convert a plastic aquarium to a tropical aquarium). All that you need is an underwater heater. If you have a centrally heated house then the heater is not going to have to work too hard, so the minimum for the size of the tank will be plentiful. But some fish keepers prefer to double up with two heaters set to the same temperature, or sometimes setting one slightly higher.
Finally you need an air supply. You can fit a venturi to the air pump that adds air to the out flow, but you must keep the filter clean. I use this on my bigger tanks, but a separate air supply on the smaller aquarium as the filters clog quicker.