If this is your first aquarium, you may want to go with a 5 gallon fish tank until you have more experience keeping fish. But setting up an aquarium, is the same, whether it's 5 gallons or 20 gallons.
A common mistake people make with smaller aquariums is overloading them with fish. When I first started raising fish. I started with a 5 gallon aquarium, and it wasn't long before I needed a larger one. So if you have the space for a larger aquarium, you may be better off getting one that will fit in the space you have.
But what makes a good aquarium? First the glass must be of high quality and sufficient thickness. Silicon joints should be neat and tidy. There should be no flaws in the glass or panels, and the edges or joints should have absolutely no damage at all. In other words, it should be solidly constructed.
The same goes for the base or stand. A gallon water ways a little over 8 pounds. So even a 5 gallon aquarium Wood weight over 40 pounds for just the tank and water. Unless you have a smaller core in, that can sit on a table or shelf, you'll need to get a stand or cabinet for your aquarium size. Naturally finding one made for your aquarium size, is the smart way to go. But you can choose other options as long as they will comfortably support the weight. Also the matter what you choose, you'll need room in back of the tank for hoses, cables, and maybe even the filter. You will also need some storage space for your supplies and may be an external canister filter.
Backgrounds are backdrops are often the last item you'll buy for your aquarium. Myself, I like to pick out the background before I choose the stones, plants, and other décor of the tank. There are many choices for background. You can simply use black plastic or paint the outside of the back panel Black. There are plastic murals of underwater scenes. My favorite, are the new ones that fit inside your tank. These are molded plastic or foam and are made to look like natural roots, rocks, and even caves.
You will also need a hood or canopy for your new aquarium. Some very good ones are now available with built-in waterproof lighting. These also usually limit ate a lot of the old problems such as for access to the aquarium. If you want to develop new interest in the hobby such as plants, the hood should hold at least two tubes, preferably more. It should also be easy to clean the aquarium without having to remove the hood entirely. Of course you will also need easy access for the cables of filters, heaters and lighting.