I agree with Slap.
And as a side note, do not fall victim to what so many others of us have fallen victim to in the past. That is, fish that we are not able to properly house.
I know when I first started keeping fish I didn't do ANY research. I just bought the fish I thought looked pretty or cool and threw them into my tank.
I soon, as have so many of us, learned that the fish selection I chose either couldn't live peacefull together, required different water parameters, or simply would just grow too big to be housed in my tank longterm.
With the vast diversity of available fish out there, no matter waht tank you have there are fish suitable to it. My absolute dream tank would consist of fish that live in my area ( a rainbow trout, smallmouth bass, channel catfish, crapie etc). I would have a tank full of one of each fish that you could readily find in a lake near me. Unfortunately I do not have the financial means to construc such an indoor facility to 1) properly house such fish and 2) do so in a way that would be asthetically pleasing to look at. Due to those facts, I am limited in the selection of fish I may keep.
The key to happy fishkeeping is to know how much $$ you have and are willing to spend on a tank of whatever size. THEN pick your fish. Even a 10g guppy tank can provide a lot of enjoyment if stocked and decorated properly. Personally, I have had as much enjoyment from watching my 2 breeding convicts as I have had from any other fish I have kept. They may not be the prettiest fish in the world, but just watching their behaviors is beautiful enough for me to keep them for as long as I am able.