So, you want to keep fish?
Do you want to keep them properly?, ie healthy and in good condition, or do you want to just look at something that is pretty and probably suffering a slow lingering death?
If your answer is the former, read on, if the latter, you are probably having difficulty reading this.
To keep fish, ie be an aquarist, you must first realise that to keep any living thing you must be responsible for its life and well being. This doesnt just mean for the time that it interests you but for ALL ITS LIFE, which will be your responsibility to make as happy and long and you possibly can.
Fish can, and quite often will, live and grow in appalling conditions, as will humans in third world countries, but we all know what we think of the people that cause this suffering!. If you do not feel you can dedicate a few hours a week  or the outlay involved in keeping them properly---DONT!. There are enough fish in this country suffering slow death already.
Right, thats got the ranting over with, if you are still reading maybe you are really interested, if so, the first thing to do is either go to the library and get a good book on fishkeeping, or purchase one. going to your local aquarist shop or "petshop" will not get you the info you require. It WILL get you a lot of expensive equiptment and fish, but experience shows that there are not many outlets now, run by people who have the knowledge or the interest in anything but selling equiptment!.
There are many points you must take into consideration, when starting to keep fish, I will list a few.
1. Can you afford the outlay.?. You cant just buy ONE tank, you will need a tank to isolate new fish in or to treat sick ones, or even to grow young fry in!. So it isnt a matter of getting one tank you will need at least two. That means two filters, two heaterstats, two lots of gravel, two hoods an lighting etc etc.! This is assuming that you want to keep them PROPERLY, and not just waste some money for a few weeks, for believe me, at some point something will go wrong, you will get bad water, a heater will go bust , either sticking on or failing, you will get disease and the fish will need treating, this is when you will need the items I am listing here.
2. Do you have the space to keep them, bearing in mind that you will need two tanks and  access to  easily clean them and do water changes etc. That lovely tank in the lounge with its expensive carpet can be a liability when it comes to a water change or even a leak!.
3. Are you willing to take the time out to LEARN about keeping them and treating their problems and diseases. To do the daily, weekly, monthly husbandry that they will require. To purchase and use regularly the water test equipt. that you will need,?
to do the weekly water changes they will require.? You may get the impression from all this that I am trying to put you OFF keeping fish, this is not so, but if I can deter the DABBLER to whom it would be only an flash in the pan, then I will be happy. These ravings were prompted after coming back into fishkeeping after a lapse of a few years to find, at least in my area, an almost total lack of interest and knowhow on the part of petshops and suppliers in the well being of the "products" that they sell, and a lack of knowledge on the part of the buyer to do anything about it.