Beginners interested in having a real aquarium with a small community of fish should start with a 10-gallon tank. There are kits available that contain everything you need to get started, minus the gravel, decorations, and fish, of course.
I believe 10-gallon tanks are good for beginners because their small size makes them easily manageable. At the same time, they are large enough to accommodate a proper filter and heater, and even some live plants. Beginners can keep a small number of community fish and learn how to care for them in a real aquarium.
This just isn’t possible with very small tanks. They often come with sub-par filters, and it is difficult to heat them. And, most fish species should not be kept in tanks under 10 gallons. This can lead to failure and frustration for the new fish keeper, and cause them to quit.
If a 10-gallon tank is out of the questions, consider a 5-gallon tank with a single betta fish. This will get you started, and you can move up from there.
Of course, there is nothing wrong with going larger if you have space. A 55-gallon tank may seem daunting, but the same principles apply as with a 10-gallon. And, because they are larger, they are a little easier to care for.