Here is some information that should help you choose a proper tank size. I like 5g tanks, here's why:
Unfortunately fish stores will sell you just about anything to get your money. And they make considerably more selling cheap tanks to a bunch of people than they do selling proper tanks to the few people who have actually done their homework on tank size.
Another common myth is that small tanks are lower maintenance. If you put three drops of red food coloring in your 1g bowl, it would probably turn the water bright red. If you put 3 drops of red food coloring in a 5g tank, it would only turn the water a light pink. The same goes for the waste your fish produces. The smaller the tank, the stronger the waste will be. So you have to clean it more often. If you are looking for a truly good beginner size, I strongly recommend you grab a 5g tank.
What’s the worst that could happen if dissolved waste reaches strong concentrations in the water? The ammonia, nitrite and nitrate given off by fish waste and rotting food is very toxic and will compromised the fish's immune system, stress him or even burn and suffocate fish in extreme cases. Therefore removal of these componds on a regular basis is essential to fish health.
Furthermore, there are no heaters on the market that will heat a 1g to the right temperature. Since it is such a small amount of water even the smallest of heaters will often make the water too hot.
A really good, economical kit is the Minibow 5 by Aqueon. The filter is nice and gentle and it comes with a full hood and light. Another great tank is the Eclipse Series by Marineland. They have many small tanks ranging from 2-5g, all of which come with an excellent in-hood 'bio-wheel' filter.