Would actually start off with knowing what your water supply hardness and ph is.
Can find that out from your water providers website, usually under water quaility or water hardness report or something akin to that.
And then look into where you would like to put a possible tank, usually in an area where there is no direct sunlight, not too much human traffic going past, not particularly near doors that gets used constantly in opening and closing throughout the day, near power supply plugs and not a million miles away from a sink, taps or bathroom for those water change days.
Once you have your possible location for a tank, you can then consider whats the biggest tank that can fit into that space, working in a company fish store will probably do you good deals there for a decent tank.
Then once you know what sized tank you can fit within the space and your budget, you can then start researching what kind of fish you would like that suits your tank size and water hardness and ph, quite important that last part.
Website Seriously Fish is pretty good for researching in to fish compatibility for your tank size and water parameters.
And finally while you're reseaching and have a tank in place etc, you can then do a fishess cycle to make the tank and filter safer for your water for your livestock to live in.
Worth reading this article on cycling tanks if you do not already know of this from working at your company fish store.
Cycling Your First Fresh Water Tank What is Cycling and Why is it Important? Fish waste, and especially fish breathing, plus uneaten food and other organic matter breaking down in a tank all produce Ammonia. This can quickly become toxic to fish if it is allowed to build up to any measurable...
www.fishforums.net
And last points, a water testing kit is pretty much essential, for testing ammonia, nitrite and nitrates, would recommend API Freshwater Master test Kit, pretty good value for money and is fairly accurate if instructions are followed properly.
https://www.amazon.co.uk/dp/B000255NCI/?tag=
And youll need a ammonia source for the fishless cycle, Dr Tims Ammonium Chloride is fine to use
https://www.amazon.co.uk/dp/B006MP4QG6/?tag=