I would really avoid a very small tank of 30 - 40 litres. They are harder to look after and the range of fish you can keep is extremely limited. A slightly larger tank takes up little more space, there is very little difference in cost and you have a much greater range of fish. Personally, I'm not keen on cube shaped tanks, since a longer but shallower tanks gives the fish a longer run for swimming. I have a Superfish Aqua Expert 70 which is only £79.99 from any branch of Maidenhead aquatics, complete with all equipment, including LED lighting. It's a very good economical first buy, costing less than most smaller tanks. It has a total volume of 70 litres, like all tanks the actual volume is smaller when you allow for gravel etc.
As far as fish are concerned, it's important to remember that almost all fish that can live in these smaller tanks are shoaling fish, they need to be kept in groups of at least 6. The big mistake many beginners make is to have either a menagerie (one of this, one of that, one of the other) or a "Noah's Ark" (two of this, two of that. two of the other). In either case, the result is conflict among fish, either within the species or between species, leading to unhappy, sick and ultimately dead fish. If you do buy shoaling fish, then you would only be able to have 2 or 3 different species in a smallish tank. I imagine you would like lively and colourful fish. Many species of Tetra would be good, some smaller barbs etc, maybe male guppies. With a tank of 60 litres or so, a small Corydoras such as the Panda cory would fit in and are great little characters. Whatever, do NOT go to a fish shop without knowing what you want, otherwise you may be tempted by something like a Clown loach, a pretty lively little fish that looks great in the shop but will grow to a foot or more long and needs to be kept in a group of 6 or more!!! Check the fish profiles on the Thinkfish web site and use the community creator to make sure your choices are compatible and you are not overstocked.